ATM communication system with high speed connection-less service function

ABSTRACT

ATM communication system capable of realizing a high speed and efficient datagram delivery for the connection-less communication among the terminals in the ATM network. In the system formed by a plurality of ATM networks inter-networking with each other, each network containing a plurality of terminals, the ATM networks with connection-less service function units for managing a connection-less datagram transmission are provided in the ATM networks, and the connection-less datagram transmission from each terminal to a destination terminal is performed by resolving a connection identifier for identifying an ATM connection connected to a destination side connection-less service function unit associated with a destination side ATM network containing the destination terminal, and transmitting datagram from said each terminal to the destination side connection-less service function unit through the ATM connection identified by the resolved connection identifier.

This application is a Continuation of application Ser. No. 08/689,920,now allowed filed on Aug. 16, 1996 now U.S. Pat. No. 5,748,626; which isa Continuation of application Ser. No. 08/456,698 filed on Jun. 1, 1995,now U.S. Pat. No. 5,583,865; which is a Divisional of application Ser.No. 08/230,539, filed on Apr. 20, 1994, now U.S. Pat. No. 5,450,406.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an ATM (Asynchronous Transfer Mode)communication system with a CLSF (Connection-Less Service Function).

2. Description of the Background Art

In order to provide the highly efficient and flexible communicationservices with respect to the increasing demands for the variety ofcommunications such as the image communication and the high speed datacommunication, there is an eager expectation for the realization of theB-ISDN (Broadband-Integrated Service Digital Network), and the ATMexchange scheme is considered as a prospective scheme for actuallyrealizing the B-ISDN.

The ATM exchange scheme is a scheme for realizing the communicationservice by loading data into a fixed length packet called cellregardless of the attributes of the data, and using this cell as a unitof exchange. The ITU (formerly CCITT) has formally determined this ATMexchange scheme as the next generation exchange scheme, and decided touse this ATM exchange scheme for realizing the B-ISDN. For this reason,it is highly likely that the demands for the next generation multi-mediacommunication and broadband communication are going to be handled byconstructing the public network or the local network based on the ATMexchange scheme.

In recent years, there is a movement for applying this ATM exchangescheme to the LAN (Local Area Network) such as the Ethernet. In thiscase, the LAN operated under the ATM exchange scheme will be referred asthe ATM-LAN. Such an ATM-LAN is expected to have the advantage that thethroughout of the LAN can be improved considerably, that it is suitablefor the multi-media, and that it is adaptive to the public network.

Now, one of the features of the ATM communication scheme is that itshigh speed operation realized by the hardware switching of the ATMcells. That is, the ATM network is the connection-oriented (CO) networkin which the virtual connection (VC) or the virtual path (VP) is set upend-to-end, and the packet called cell is delivered end-to-end by labelmultiplexing or label exchanging the VCs or VPs in terms of theiridentifiers (VCI or VPI).

The data to be delivered end-to-end is loaded in the payload section ofthe ATM cell, and the ATM cell is exchanged and transmitted up to thedestination terminal by the hardware switching operation alone withoutthe intervention of the software operation, where the hardware switchingoperation is carried out by the ATM switch according to the VPI/VCI (orthe value of the other field such as PT in the ATM cell header)contained in the ATM cell header.

In contrast to this ATM communication scheme which is theconnection-oriented communication scheme, the communication scheme usedin the conventional data communication is the connection-less (CL)communication scheme in which the end-to-end connections are notnecessarily set up, and the packet is transmitted to the destinationterminal as the packet is sent out to the network by attaching thedestination data as its part while some node in the network analyzes thedestination data and carries out the routing processing. Namely, in theconnection-less communication scheme, the data transmission is realizedwithout the procedure for setting up the connections at the terminals.In such a case, the packet to be transmitted to the destination terminalin connection-less manner is called datagram and this data transmissionis called the datagram transmission. Thus, in the connection-lesscommunication scheme, the communication is realized in a form of thedatagram transmission without the procedure for setting up theconnections.

Almost all of the existing data terminals such as the workstation (WS)and the personal computer (PC) adopts this datagram transmission schemebecause the datagram transmission scheme is supported by the LAN, andthe software provided within the data terminals such as protocols TCP/IPand UDP/IP has been suitable for the datagram transmission.

In such existing terminals or terminals provided with the existingprotocols, i.e., the terminal which generates the datagram and outputsit to the destination terminal/network through the ATM network, thedatagram transmission scheme is used for the terminal to terminalcommunication. To this end, it is necessary for the terminal and thenetwork to modified to realize the function for adapting the terminal tothe interface with respect to the ATM-LAN by replacing the usual LANboard with the ATM board such as the Ethernet board or by using theterminal adaptor (TA), the function for loading the datagram into theATM cell somehow at the terminal, and the function to deliver thedatagram to the destination terminal indicated by the destinationaddress at the network. Here, the terminal include the gate-way betweenthe existing LAN and the ATM network.

To realize these functions, the datagram delivery scheme using the CLSFhas been used conventionally. In this datagram delivery scheme, the CLSFprocessing unit is provided within the ATM network, and all thedatagrams are collected there once. In other words, the CLSF processingunit is connected with all the datagram terminals by PVC (Semi-PermanentVC) (or VC, VP, PVC, or PVP), and the terminal wishing to transmit thedatagrams assembles the ATM cells for all the datagrams to betransmitted, and transmits the ATM cells to the VC directed toward theCLSF processing unit. The CLSF processing unit then reproduces thereceived datagrams, and selects the VC connected to the destinationaddress by analyzing the destination address of the datagrams, and thenre-assembles the ATM cells for the datagrams and transmits the ATM cellsto the selected VC. In a case the VC connected to the destinationaddress cannot be found while there are other CLSF processing unitswithin the network, the CLSF processing unit transmits the re-assembledATM cells to the next stage CLSF processing unit which is expected tocontain the terminal with the destination address or which is determinedby the routing rule in advance.

Here, it is not absolutely necessary for the CLSF processing unit toanalyze the destination address after reproducing the datagrams, andtransmits the ATM cells after re-assembling the ATM cells. For instance,in a case the destination address is contained in the to cell among theATM cells for the datagrams, the destination address of the first cellalone can be analyzed and then transmitted to the destination terminal,and then the subsequent cells of the ATM cells for the datagrams can besequentially transmitted to that destination terminal.

However, in this datagram delivery scheme using CLSF, all the datagramsoriginating within the network are always going to be transmitted viathe CLSF processing unit, so that the CLSF processing unit is requiredto have a higher throughput as the amount of datagrams to be transmittedincreases and as the number of terminals within the network increases.Consequently, the CLSF processing unit is required to have a very highthroughput and the flexibly expandability.

Another scheme for transmitting the datagrams to the destinationterminal is to set up the ATM connection such as VC to the destinationaddress, and the ATM cells for the datagrams are delivered through thisVC. However, in this scheme, there is a serious problem concerning theselection of the destination terminal with respect to which the VC is tobe set up. Namely, there are enormously many terminals to which thedatagrams can possibly be transmitted in practice, and in addition thegeneration of the datagrams is more bursty compared with the speechdata, etc., so that to set up the enormously many connections is goingto be a considerable waste of the network resource.

Furthermore, in a case of realizing the connection-less communication inthe conventional ATM network, the ATM connection is always terminated atthe CLSF processing unit, and the protocol processing for the upperlayers above the AAL layer such as the protocol for the connection-lessservice called CLNAP (Connection-Less Network Access Protocol) iscarried out. In other words, even in a case of the datagram transmissionbetween the quite nearby terminals, the ATM connection is going to beterminated once at the CLSF processing unit. Also, in a case of thedatagram transmission between far distanced terminals, it becomesnecessary to pass through a plurality of CLSF processing units at eachone of which the protocol processing above the AAL layer must be carriedout.

In general, the protocol processing above the AAL layer such as theCLNAP is realized by the software processing so that the processingspeed is slow compared with the processing below the AAL layer which isusually carried out be the hardware processing. Also, it is necessaryfor the CLSF processing unit to carry out the analysis of the addressdata such as the network layer address data in the datagrams for notjust the transmissions to the terminals of the network supported by thatCLSF processing unit itself but also for the transmissions to theterminals of the network supported by the other CLSF processing unit aswell, so that the datagram transmission processing load is going to beconcentrated on the CLSF processing unit. For these reasons, it has beendifficult to realize the high speed communication in the connection-lesscommunication (datagram delivery) among the terminals of theconventional ATM communication system.

On the other hand, in making the inter-LAN connection, i.e., theinter-networking among the LANs, in the conventional LAN environment,the router has been required to be provided between each adjacent LANs.The main function of this router is the routing processing for thedatagram transmission over the LANs, by processing up to the layer 3(network layer) in the OSI (Open Systems Interconnection) protocol layerstack. Namely, for the datagram to be transmitted over two LANs, thedatagram must be brought up to the layer 3 by the router to analyze thedestination network layer address there, and the delivered to thedestination LAN according to the result of this analysis. The functionof this router also realized by the so called "gate-way" in the contextof the computer communication, but the "gate-way" is formally defined asthat which carries out the processing up to the layer 7, so that theelement for realizing this function will be called router in thefollowing.

There is also an element called "bridge" which has the similar functionas the router in realizing the inter-LAN connection. In this bridge, incontrast to the router which determines the destination LAN by analyzingthe destination network layer address, the destination LAN is determinedby analyzing the data link layer address (MAC address). Namely, thebridge realized the inter-LAN connection by analyzing the destinationMAC address of the datagram and passing the datagram through to anotherLAN when the obtained MAC address is not destines within its own LAN.

Furthermore, there is also a similar element called "brouter" whichfunctions as the router for the predetermined network layer protocol andas the bridge for all the other protocols.

These router, bridge, and brouter has been usually realized by theworkstation (WS). Namely, the CPU provided within the WS carries out theaddress analysis and realized the functions of the router, bridge, andbrouter by transmitting the datagram to the allocated physical port.

However, in a case of the ATM-LAN, these router, bridge, and brouter aregoing to terminate the connection at the layer 3 or the layer 2forcefully and the processing for the layer 3 and layer 2 after thetermination is most likely handled by the software processing. For thisreason, for the transmission over the LANs, the speed and the capacityof the communication can be considerably lowered compared with thecommunication within the LAN. Also, in a case of providing the router,bride, brouter, etc., the VP/VC cannot be set up over the LANs becausethe layer processing above the ATM layer between the end points iscarried out at the routers.

Thus, in the conventional ATM-LAN, the layer 3 (network layer)processing must be carried out at the physical boundary of the networks,because the physical network boundary is the boundary of the OSI layer 2(data link layer), and therefore the router must be provided at thephysical network boundary for this reason.

Moreover, the conventional routing protocol to be executed by the routercannot be executed correctly unless the router is located within thephysical network to which it belongs.

As a consequence, the location of the router has been dictated by thephysical configuration of the network conventionally, i.e., the topologyof the network layer cannot be defined independently from the topologyof the physical network. In addition, it has been impossible to locatethe router belonging to a certain network outside of that certainnetwork.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an ATMcommunication system capable of realizing a high speed and efficientdatagram delivery for the connection-less communication among theterminals in the ATM network.

It is another object of the present invention to provide an ATMcommunication system capable of setting the topology of the networklayer independently from the topology of the physical network.

According to one aspect of the present invention there is provided anATM communication system, comprising: a plurality of ATM networksinter-networking with each other, each network containing a plurality ofterminals; and connection-less service function means for managing aconnection-less datagram transmission in the ATM networks; wherein theconnection-less datagram transmission from each terminal to adestination terminal is performed by resolving a connection identifierfor identifying an ATM connection connected to a destination sideconnection-less service function means associated with a destinationside ATM network containing the destination terminal, and transmittingdatagram from said each terminal to the destination side connection-lessservice function means through the ATM connection identified by theresolved connection identifier.

According to another aspect of the present invention there is providedan ATM communication system, comprising: a plurality of ATM networksinter-networking with each other, each network containing a plurality ofterminals and the ATM networks including a first ATM network havingconnection-less service function means for managing a connection-lessdatagram transmission in the ATM networks, and a second ATM networkhaving no connection-less service function means; inter-networking meansfor inter-networking the first and second ATM networks; and connectionset up means for setting up a first ATM connection between theconnection-less service function means of the first ATM network and theinter-networking means, and a second ATM connection between theinter-networking means and a terminal belonging to the second ATMnetwork; wherein the inter-networking means directly connects the firstand second ATM connections set up by the connection set up means at anATM layer, and the connection-less service function means of the firstATM network is assigned with an address data indicating that saidconnection-less service function means logically belongs to the secondATM network at a network layer, such that the connection-less datagramtransmission from said terminal belonging to the second ATM network isperformed by using said address data through the first and second ATMconnections connected at the ATM layer.

According to another aspect of the present invention there is providedan ATM communication system, comprising: a plurality of ATM networksinter-networking with each other, each network containing a plurality ofterminals and the ATM networks including a first ATM network havingconnection-less service function means for managing a connection-lessdatagram transmission in the ATM networks, and second and third ATMnetworks having no connection-less service function means; firstinter-networking means for inter-networking the first and second ATMnetworks; second inter-networking means for inter-networking the secondand third ATM networks; and connection set up means for setting up afirst ATM connection between the connection-less service function meansof the first ATM network and the inter-networking means, a second ATMconnection between the inter-networking means and the secondinter-networking means, and a third ATM connection between the secondinter-networking means and a terminal belonging to the third ATMnetwork; wherein the inter-networking means directly connects the first,second, and third ATM connections set up by the connection set up meansat an ATM layer, and the connection-less service function means of thefirst ATM network is assigned with an address data indicating that saidconnection-less service function means logically belongs to the thirdATM network at a network layer, such that the connection-less datagramtransmission from said terminal belonging to the third ATM network isperformed by using said address data through the first, second, andthird ATM connections connected at the ATM layer.

According to another aspect of the present invention there is provided amethod for ATM communication in an ATM communication system formed by aplurality of ATM networks inter-networking with each other, each networkcontaining a plurality of terminals, the method comprising the steps of:providing the ATM networks with connection-less service function meansfor managing a connection-less datagram transmission in the ATMnetworks; and performing the connection-less datagram transmission fromeach terminal to a destination terminal by resolving a connectionidentifier for identifying an ATM connection connected to a destinationside connection-less service function means associated with adestination side ATM network containing the destination terminal, andtransmitting datagram from said each terminal to the destination sideconnection-less service function means through the ATM connectionidentified by the resolved connection identifier.

According to another aspect of the present invention there is provided amethod of ATM communication in an ATM communication system formed by aplurality of ATM networks inter-networking with each other, each networkcontaining a plurality of terminals and the ATM networks including afirst ATM network having connection-less service function means formanaging a connection-less datagram transmission in the ATM networks,and a second ATM network having no connection-less service functionmeans, the method comprising the steps of: inter-networking the firstand second ATM networks by first inter-networking means; setting up afirst ATM connection between the connection-less service function meansof the first ATM network and the inter-networking means, and a secondATM connection between the inter-networking means and a terminalbelonging to the second ATM network; directly connecting the first andsecond ATM connections at an ATM layer by the inter-networking means;assigning the connection-less service function means of the first ATMnetwork with an address data indicating that said connection-lessservice function means logically belongs to the second ATM network at anetwork layer, such that the connection-less datagram transmission fromsaid terminal belonging to the second ATM network is performed by usingsaid address data through the first and second ATM connections connectedat the ATM layer.

According to another aspect of the present invention there is provided amethod of ATM communication in an ATM communication system formed by aplurality of ATM networks inter-networking with each other, each networkcontaining a plurality of terminals and the ATM networks including afirst ATM network having connection-less service function means formanaging a connection-less datagram transmission in the ATM networks,and second and third ATM networks having no connection-less servicefunction means, the method comprising the steps of: inter-networking thefirst and second ATM networks by first inter-networking means, and thesecond and third ATM networks by second inter-networking means; settingup a first ATM connection between the connection-less service functionmeans of the first ATM network and the inter-networking means, a secondATM connection between the inter-networking means and the secondinter-networking means, and a third ATM connection between the secondinter-networking means and a terminal belonging to the third ATMnetwork; directly connecting the first, second and third ATM connectionsat an ATM layer by the first and second inter-networking means; andassigning the connection-less service function means of the first ATMnetwork with an address data indicating that said connection-lessservice function means logically belongs to the third ATM network at anetwork layer, such that the connection-less datagram transmission fromsaid terminal belonging to the third ATM network is performed by usingsaid address data through the first, second, and third ATM connectionsconnected at the ATM layer.

Other features and advantages of the present invention will becomeapparent from the following description taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a first type ATM network block in theATM communication system according to the present invention.

FIG. 2 is a schematic block diagram of an inter-networking unit in thefirst type ATM network block of FIG. 1.

FIG. 3 is a schematic diagram of a second type ATM network block in theATM communication system according to the present invention.

FIG. 4 is a schematic block diagram of an inter-networking unit in thesecond type ATM network block of FIG. 3.

FIG. 5 is a partial network diagram of the ATM communication systemaccording to the present invention showing a first case of the datagramdelivery within sub-network.

FIG. 6 is a data exchange diagram for the first case of the datagramdelivery within sub-network shown in FIG. 5.

FIG. 7 is a flow chart for an operation in the first case of thedatagram delivery within sub-network shown in FIG. 5.

FIG. 8 is a partial network diagram of the ATM communication systemaccording to the present invention, showing an alternative procedure forthe first case of the datagram delivery within sub-network.

FIG. 9 is a data exchange diagram for the alternative procedure for thefirst case of the datagram delivery within sub-network shown in FIG. 8.

FIG. 10 is a partial network diagram of the ATM communication systemaccording to the present invention, showing a second case of thedatagram delivery within sub-network.

FIG. 11 is a schematic overall network diagram of the ATM communicationsystem according to the present invention in a case of the hierarchicalnetwork topology.

FIG. 12 is a schematic network diagram of the network of FIG. 11 for anaddress resolution in a first case of the datagram delivery to externalnetwork.

FIG. 13 is a schematic network diagram of the network of FIG. 11 for adatagram transmission in a first case of the datagram delivery toexternal network.

FIG. 14 is a schematic diagram of an address space view from one addressresolution server in the case of the datagram delivery to externalnetwork shown in FIG. 12.

FIG. 15 is a schematic diagram of an address space view from anotheraddress resolution server in the case of the datagram delivery toexternal network shown in FIG. 12.

FIG. 16 is a schematic network diagram of the network of FIG. 11 for analternative procedure for an address resolution in a first case of thedatagram delivery to external network.

FIG. 17 is a schematic diagram of an address space view from one addressresolution server in the case of the datagram delivery to externalnetwork shown in FIG. 16.

FIG. 18 is a schematic diagram of an address space view from anotheraddress resolution server in the case of the datagram delivery toexternal network shown in FIG. 16.

FIG. 19 is a diagrammatic illustration of a VCI/VPI reqriting table usedin the case of the datagram delivery to external network shown in FIG.13.

FIG. 20 is a diagram of data layer sequence for a protocol processing inthe case of the datagram delivery to external network shown in FIG. 13.

FIG. 21 is a schematic network diagram of the network of FIG. 11 for oneexemplary case of the datagram delivery to external network.

FIG. 22 is a schematic network diagram of the network of FIG. 11 foranother exemplary case of the diagram delivery to external network.

FIG. 23 is a diagram for one procedure of an address data management ina second case of the datagram delivery to external network.

FIG. 24 is a diagram for another procedure of an address data managementin a second case of the datagram delivery to external network.

FIG. 25 is a diagram for still another procedure of an address datamanagement in a second case of the datagram delivery to externalnetwork.

FIG. 26 is a diagram of ATM connections in a second case of the datagramdelivery to external network.

FIG. 27 is a schematic overall network diagram of the ATM communicationsystem according to the present invention in a third case of thedatagram delivery to external network.

FIG. 28 is a diagram for one procedure of an address data management inthe third case of the datagram delivery to external network shown inFIG. 27.

FIG. 29 is a diagram for another procedure of an address data managementin the third case of the datagram delivery to external network shown inFIG. 27.

FIG. 30 is a schematic network diagram of the network of FIG. 27 for adatagram transmission in the third case of the datagram delivery toexternal network.

FIG. 31 is a schematic diagram of connection-less service functionprocessing units in the network of FIG. 27.

FIG. 32 is a diagram of data layer sequence for a protocol processing inthe third case of the datagram delivery to external network shown inFIG. 27.

FIG. 33 is a diagram of ATM connections in the third case of thedatagram delivery to external network.

FIG. 34 is a schematic overall network diagram of the ATM communicationsystem according to the present invention in a case of the flat networktopology.

FIG. 35 is a schematic network diagram of the network of FIG. 34 for anexemplary datagram transmission in a first case of the datagram deliveryto external network.

FIG. 36 is a diagram of data layer sequence for a protocol processing inthe first case of the datagram delivery to external network shown inFIG. 35.

FIG. 37 is a schematic network diagram of the network of FIG. 34 foranother exemplary datagram transmission in a first case of the datagramdelivery to external network.

FIG. 38 is a diagram of data layer sequence for a protocol processing inthe first case of the datagram delivery to external network shown inFIG. 37.

FIG. 39 is a diagram of ATM connections in the first case of thedatagram delivery to external network shown in FIG. 37.

FIG. 40 is a schematic network diagram of the network of FIG. 34 forstill another exemplary datagram transmission in a first case of thedatagram delivery to external network.

FIG. 41 is a diagram of data layer sequence for a protocol processing inthe first case of the datagram delivery to external network shown inFIG. 40.

FIG. 42 is a diagram of ATM connections in the first case of thedatagram delivery to external network shown in FIG. 40.

FIG. 43 is a schematic network diagram of the network of FIG. 34 for anexemplary datagram transmission in a second case of the datagramdelivery to external network.

FIG. 44 is a diagram of data layer sequence for a protocol processing inthe second case of the datagram delivery to external network shown inFIG. 44.

FIG. 45 is a schematic network diagram of the network of FIG. 34 foranother exemplary datagram transmission in a second case of the datagramdelivery to external network.

FIG. 46 is a diagram of ATM connections in the second case of thedatagram delivery to external network shown in FIG. 45.

FIG. 47 is a schematic network diagram of the network of FIG. 34 forstill another exemplary datagram transmission in a second case of thedatagram delivery to external network.

FIG. 48 is a diagram of ATM connections in the second case of thedatagram delivery to external network shown in FIG. 47.

FIG. 49 is a schematic overall network diagram of the ATM communicationsystem according to the present invention in a case of a large scalenetwork architecture.

FIG. 50 is a schematic overall network diagram of the network of FIG. 49showing the sub-networks involved.

FIG. 51 is a schematic network diagram of the network of FIG. 50 for adatagram transmission in a case of the datagram delivery to externalnetwork.

FIG. 52 is a diagram of ATM connections for an exemplary datatransmission in a first case of the datagram delivery to externalnetwork shown in the network of FIG. 50.

FIG. 53 is a diagram of ATM connections for another exemplary datatransmission in a first case of the datagram delivery to externalnetwork shown in the network of FIG. 50.

FIG. 54 is a diagram of ATM connections for an exemplary datatransmission in a second case of the datagram delivery to externalnetwork shown in the network of FIG. 50.

FIG. 55 is a diagram of ATM connections for another exemplary datatransmission in a second case of the datagram delivery to externalnetwork shown in the network of FIG. 50.

FIG. 56 is a schematic network diagram of the ATM communication systemaccording to the present invention for one embodiment in a case of amodified network layer topology.

FIG. 57 is a schematic block diagram of an inter-networking unit in thenetwork of FIG. 56.

FIG. 58 is a flow chart for an operation in the network of FIG. 56.

FIG. 59 is a partial schematic diagram of the network of FIG. 56 showingthe logically connected region.

FIG. 60 is a network layer address form in the network of FIG. 56.

FIG. 61 is a schematic network diagram of another network configurationin the ATM communication system according to the present invention in acase of a modified network layer topology.

FIG. 62 is a schematic network diagram of the ATM communication systemaccording to the present invention for an execution of a routingprotocol in a case of a modified network layer topology.

FIG. 63 is a flow diagram for the execution of the routing protocolshown in FIG. 62.

FIG. 64 is a schematic network diagram of the ATM communication systemaccording to the present invention for a transmission of a routing datain a case of a modified network layer topology.

FIG. 65 is a schematic network diagram of another configuration of theATM communication system according to the present invention for oneembodiment in a case of a modified network layer topology.

FIG. 66 is a flow chart for an operation in the network of FIG. 65.

FIG. 67 is a schematic network diagram of the ATM communication systemaccording to the present invention for another embodiment in a case of amodified network layer topology.

FIG. 68 is a schematic diagram of one ATM connection setting in the ATMcommunication system according to the present invention for anotherembodiment in a case of a modified network layer topology.

FIG. 69 is a schematic diagram of another ATM connection setting in theATM communication system according to the present invention for anotherembodiment in a case of a modified network layer topology.

FIG. 70 is a cell header form in the network of FIG. 69.

FIG. 71 is a schematic diagram of one configuration of the ATMcommunication system according to the present invention for carrying outthe broadcast in a case of a modified network layer topology.

FIG. 72 is a schematic diagram of another configuration of the ATMcommunication system according to the present invention for carrying outbroadcast in a case of a modified network layer topology.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

ATM Network Block Configuration

The ATM communication system according to the present invention can beconstructed from one or more of the following ATM network blocks.

1. The first type ATM network block

FIG. 1 shows a first type ATM network block in the ATM communicationsystem according to the present invention, which comprises a firstATM-LAN 11 and a second ATM-LAN 12 which are connected through an IWU(inter-networking unit) 13, where each of the first ATM-LAN 11 and thesecond ATM-LAN 12 is a local area network formed by a plurality ofterminals and nodes operated by the ATM scheme and equipped with aconnection-less service function processing unit (CLSF) 14 or 15,respectively.

In this ATM network block of FIG. 1, each ATM-LAN has an independentaddress assignment policy within itself. Namely, the right to determineVPI/VCI used within each ATM-LAN are assigned to a VPI/VCI determinationfunction provided within each ATM-LAN, and this right is assignedindependently for each ATM-LAN.

In a case of a presence of a data to be transmitted, regardless ofwhether the destination of the data is within the same ATM-LAN or not, aterminal and nodes within each ATM-LAN transmits that data within theATM-LAN by loading that data in an ATM cell and attaching an appropriateATM cell header.

In this ATM network block of FIG. 1, the IWU 13 has a detailed internalconfiguration as shown in FIG. 2 which comprises: an add/drop processingunit 21 provided on a cell transmission path 20, amultiplexer/demultiplexer (MUX/DEMUX) 22 having its output connectedwith the add/drop processing unit 22, a call processing unit 24 and anIWU management unit 25 which are connected with inputs of the MUX/DEMUX22, and an ATM cell header conversion unit 26 also provided on the celltransmission path 20. Here, this IWU 13 is located between the twoATM-LANs 11 and 12 and functions to control the inter-networking(inter-LAN connection) between these two ATM-LANs 11 and 12.

The add/drop processing unit 21 looks up the header portion of the ATMcell entering to it, and in a case that cell has the appropriate headervalue, i.e., in a case that cell is a cell to be terminated within theIWU 13, it executes the processing for dropping that cell to the DEMUX22 side, and the processing for adding the cell from the MUX 22 sideonto the cell transmission path 20. Here, the add/drop processing unit21 in this ATM network block of FIG. 1 is capable of adding or droppingthe cell to either one of the right and left directions of the celltransmission path 20.

In this add/drop processing unit 21, which one of the right and leftdirections of the cell transmission path 20 should the cell be added isspecified by the call processing unit 24 and the IWU management unit 25as described below. Namely, along with the cell to be added (add cell),the add/drop processing unit 21 receives a value "0" in a case of addingthat cell to the left direction of the cell transmission path 20, or avalue "1" in a case of adding that cell to the right direction of thecell transmission path 20, such that the direction for adding that cellcan be determined at the add/drop processing unit 21 according to thereceived value. As for the position for adding the cell, it may bepossible to add the cell in replacement of an empty cell passing overthe cell slot on the cell transmission path 20.

On the other hand, in a case of dropping the cell to the MUX/DEMUX 22side, the add/drop processing unit 21 looks up a prescribed drop tableprovided for each of the right and left directions of the pathtransmission path 20 which registers the cell header value incorrespondence to the dropping target, such that the add/drop processingunit 21 can determined the dropping target for the cell as one of thecall processing unit 24 and the IWU management unit 25 according to thecell header value of the cell and this drop table. This drop table isinitialized and managed by the IWU management unit 25 as describedbelow. Here, for example, the drop target value "1" could be assigned tothe call processing unit 24 while the drop target value "2" could beassigned to the IWU management unit 25 and a cell (drop cell) dropped atthe add/drop processing unit 21 is transmitted to the MUX/DEMUX 22 sidealong with the drop target value and a value for indicating which one ofthe right and left directions of the cell transmission path 20 has thatcell been dropped from ("0" for the left direction for "1" for the rightdirection for example).

The MUX/DEMUX 22 has a DEMUX function to transmit the drop celltransmitted from the add/drop processing unit 21 to an appropriate oneof the connected modules (the call processing unit 24 and the IWUmanagement unit 25) according to the data accompanying the drop cell,and a MUX function to transmit the add cell transmitted from theconnected modules to the add/drop processing unit 21 side along with thedata indicating which one of the right and left directions of the celltransmission path 20 should the add cell to be added.

The call processing unit 24 has basic functions for setting up, cuttingoff, changing, and managing the ATM connection over the IWU 13. Inaddition, this call processing unit 24 may also has additional functionsfor managing the bandwidth of the ATM connection or the celltransmission path 20 within the IWU 13.

The IWU management unit 25 has functions for managing and controllingthe IWU 13.

The ATM cell header conversion unit 26 looks up the header value of thecell entering to it, and in a case the header value is an appropriatevalue, it rewrites this header value (input cell header value) toanother value (output cell header value). For this purpose, the ATM cellheader conversion unit 26 has a correspondence table registering theinput cell header values and the output cell header values incorrespondence. This correspondence table is also initialized andmanaged by the IWU management unit 25. As this correspondence tablecontains the input cell header value which is also registered in thedrop table mentioned above, the correspondence table and the drop tablecan be unified together into a single table easily.

In this IWU 13, the modules are connected with each other through a busline (now shown), and the control from the IWU management unit 25 suchas the changing of the setting value at each module is achieved throughthis bus line. Alternatively, the data exchange among the modules may beachieved by loading the data to be exchanged into the ATM cell andexchanging the ATM cell among the modules, instead of using the busline, if desired.

It is also possible to separate each module into two parts correspondingto the right and left directions of the cell transmission path 20,instead of handling the processing for both directions in the singlemodule as in the above.

It is also possible to combine a part of a whole of the variousprocessings to be executed by the modules into an integrated processingto be executed by a single CPU/MPU.

Also, the ATM cell header conversion unit 26 may be provided on both ofthe right and left sides of the add/drop processing unit 21 in order tohandle the ATM cell header conversion for the ATM cell flow in the rightand left directions of the cell transmission path 20 separately.Alternatively, the add/drop processing unit 21 may be provided on bothof the right and left sides of the ATM cell header conversion unit 26 inorder to handle the add/drop processing for the ATM cell flow in theright and left directions of the cell transmission path 20 separately.In these cases, the functions of the IWU 13 can be made to appearsymmetrical for the right and left directions of the cell transmissionpath 20.

In this ATM network block of FIG. 1, the CLSF 14 (15) realizes theconnection-less service function in which the datagram delivered overthe IWU 13 is terminated once, and after the network layer address islooked up, the datagram is transmitted to the appropriate ATMconnection. Thus, the ATM connection for the datagram delivery isterminated once at the CLSF 14 (15), but there is no need to re-assemblethe datagram. That is, at the CLSF 14 (15), it is not necessary to makethe network layer termination, and it suffices to make theconnection-less (CL) layer termination as considered by the CCITT. Inthis case, the appropriate ATM connection to which the datagram istransmitted from the CLSF 14 (15) is the VP/VC connected with theterminal/node having the looked up network layer address, or the VP/VCconnected with the other CLSF which is expected to have the function ofthe datagram delivery to the looked up network layer address.

2. The second type ATM network block

FIG. 3 shows a second type ATM network block in the ATM communicationsystem according to the present invention, which comprises a firstATM-LAN 131, a second ATM-LAN 132, and a third ATM-LAN 133 which areconnected through an IWU (inter-networking unit) 134, where each of thefirst ATM-LAN 131, the second ATM-LAN 132, and the third ATM-LAN 133 isa local area network formed by a plurality of terminals and nodesoperated by the ATM scheme and equipped with a connection-less servicefunction processing unit (CLSF) 135, 136, or 137, respectively.

In this ATM network block of FIG. 3, similarly to the first type ATMnetwork block of FIG. 1 described above, each ATM-LAN has an independentaddress system within itself, and in a case of a presence of a data tobe transmitted, regardless of whether the destination of the data iswithin the same ATM-LAN or not, a terminal and nodes within each ATM-LANtransmits that data within the ATM-LAN by loading that data in an ATMcell and attaching an appropriate ATM cell header.

In this ATM network block of FIG. 3, the IWU 134 has a detailed internalconfiguration as shown in FIG. 4 which comprises: an ATM switch 141 of Ninputs and M outputs (N=M=2 of FIG. 4), a call processing unit 143, anIWU management unit 144, input processing units 14A and 14B for enteringinputs from two other ATM-LANs, and output processing units 14X and 14Yfor outputting outputs to two other ATM-LANs, where the ATM switch 141switches the outputs of the input processing units 14A and 14B, the callprocessing unit 143, and the IWU management unit 144 into the inputs ofthe output processing units 14X and 14Y, the call processing unit 143,and the IWU management unit 144. In a general case of having more thanthree ATM-LANs, this IWU 134 can be expanded by incorporating more thantwo input processing units and the output processing units, andfunctions to control the inter-networking (inter-LAN connection) amongthese ATM-LANs.

Each of the input processing units 14A and 14B has functions to analyzethe header value such as VPI/VCI value of the ATM cell entering to it,carry out the conversion of the header value if necessary, and attachinga routing tag for enabling an appropriate routing of the cell at the ATMswitch 141.

The ATM switch 141 then carries out the routing of the cell entered fromthe input processing unit 14A or 14B according to the routing tagattached to the cell. In addition, the ATM switch 141 may also have thefunctions such as the broadcast function or the multicast function ifdesired.

Each of the output processing units 14X and 14Y has functions to removethe routing tag from the cell outputted from the ATM switch 141, andcarries out the conversion of the header value if necessary. This latterATM cell header conversion function is a function required to beprovided either at the input processing unit or the output processingunit such that a set of the input processing unit and the outputprocessing unit can control the connection with respect to the switchingnodes and terminals within the neighboring ATM-LAN, or even to the IWU134 if necessary.

The call processing unit 143 has basic functions for setting up, cuttingoff, changing, and managing the ATM connection over the IWU 134,similarly to the call processing unit 24 in the configuration of FIG. 1described above.

The IWU management unit 144 has functions for managing and controllingthe IWU 134, similarly to the IWU management unit 25 in theconfiguration of FIG. 1 described above.

In this IWU 134, the modules are connected with each other through a busline (not shown), and the control from the IWU management unit 144 suchas the changing of the setting value at each module is achieved throughthis bus line. Alternatively, the data exchange among the modules may beachieved by loading the data to be exchanged into the ATM cell andexchanging the ATM cell among the modules, instead of using the busline, if desired.

It is also possible to combine a part of a whole or the variousprocessings to be executed by the modules into an integrated processingto be executed by a single CPU/MPU.

In this ATM network block of FIG. 3, the CLSF 135 (136, 137) realizesthe connection-less service function similar to that realized by theCLSF 14 (15) in the configuration of FIG. 1 described above.

Datagram Delivery Within Sub-Network

When the ATM communication system according to the present invention isconstructed by connecting a plurality of the ATM network blocks asdescribed above, each ATM-LAN can be regarded as a sub-network. Here, ascheme for the datagram delivery between terminals within such asub-network will be described.

1. General ATM network

In an exemplary sub-network 410 shown in FIG. 5 which is equipped withthe address resolution server (ARS) 413 and the CLSF 414 and which isconnected with the external network 416 through the IWU 415, oneprocedure for the datagram delivery from a terminal (TE) 411 to anotherterminal 412 can be carried out as follows.

Namely, when a datagram transmission request occurs at the terminal 411,the terminal 411 makes an address resolution (AR) request for obtaininga suitable ATM address for the datagram transmission to a destinationterminal 412 with respect to the ARS 413 through an ATM connection 41A.

In response, the ARS 413 notifies an address resolution (AR) responseindicating VCI/VPI data or ATM address (as defined by the ATM Forum) tobe attached to the datagram at the terminal 411 which are identifiers ofan appropriate ATM connection for the datagram transmission to thedestination terminal 412, through an ATM connection 41B to the terminal411.

When this AR response is received at the terminal 411, the notifiedVCI/VPI are attached to the datagram and the datagram is outputted tothe network. The outputted datagram is then directly delivered to thedestination terminal 412 through an ATM connection 41C specified by theattached VCI/VPI of the datagram.

In this case, there is a need to provide full meshed ATM connectionsamong all the terminals within each sub-network. If ATM address isreplied by ARS, the terminal will establish ATM connection for thedestination terminal using the informed ATM address.

In this procedure, the AR request can be made either whenever thedatagram transmission request has occurred, or only when an appropriateaddress data for the destination terminal cannot be found in an addressresolution (AR) table provided in the terminal 411 which is a cachememory for the destination targets. In the latter case, when the addressof the destination targets. In the latter case, when the address of thedestination terminal can be found in the AR table, the procedure formaking the AR request and receiving an address resolution (AR) responsefrom the ARS 413 can be omitted.

Thus, in this case, the datagram delivery is achieved by the dataexchange among the ARS 413 and the terminals 411 and 412 as representedin FIG. 6, where the datagram 421 is a case in which the address datafor the destination terminal 412 cannot be found in the AR table at theterminal 411 so that the datagram transmission is preceded by theexchange of the AR request and the AR response between the ARS 413 andthe terminal 411, whereas the datagram 422 is a case in which theaddress data for the destination terminal 412 can be found in the ARtable at the terminal 411 so that the datagram transmission can takeplace immediately without the exchange of the AR request and the ARresponse.

Also, in this case, the terminal 411 carries out the protocol accordingto the flow chart of FIG. 7, in which after the datagram transmissionrequest occurs at the step 431, whether the address resolution ispossible by using the AR table of this terminal 411 or not is checked atthe step 432. If so, the datagram transmission takes place immediatelyat the step 433, whereas otherwise the AR request is made to the ARS 413at the step 434, the AR response is received from the ARS 413 at thestep 435, and then the datagram transmission takes place at the step436.

It is to be noted here that the exchange of the AR request and the ARresponse can be realized by using the point-to-point ATM connections asshown in FIG. 5, or by using a broadcast channel.

FIG. 8 shows a case of another procedure for the datagram delivery fromthe terminal 411 to the terminal 412 which can be carried out asfollows.

Namely, this is a case in which the actual datagram transmission iscarried out be the CLSF 414. In this case, when a datagram transmissionrequest occurs at the terminal 411, the terminal 411 carries out theaddress resolution to obtain the ATM connection data for the datagramtransmission to the destination terminal 412 according to the AR tableprovided in this terminal 411. In a case the address data for thedestination terminal 412 cannot be found in the AR table at the terminal411, the terminal 411 makes the AR request for obtaining a suitable ATMaddress for the datagram transmission to a destination terminal 412 withrespect to the ARS 413 through an ATM connection 41A.

In response, in a case the destination terminal is a terminal in thissub-network 410, the ARS 413 notifies the AR response indicating ATMlayer address data through an ATM connection 41B to the terminal 411. Acase in which the destination terminal is not a terminal in thissub-network 410 will be described below.

When the address resolution is completed, the terminal 411 attaches theappropriate VCI/VPI either obtained from the AR table or the AR responseto the cell for transmitting the datagram and the cell is outputted tothe network. Here, the attached VCI/VPI are the identifiers of an ATMconnection 441 from the terminal 411 to the CLSF 414, so that theoutputted cell reaches to the CLSF 414 through this ATM connection 441.At the CLSF 414, the address data of the received datagram is analyzed,and the VCI/VPI of an ATM connection 442 for transmitting the datagramto the destination terminal 412 are attached to the cell, and the cellis outputted to the network. The outputted cell then reaches to thedestination terminal 412 through the ATM connection 442.

In this alternative procedure, there is a need to provide star-shapedATM connections from the CLSF 414 to all the terminals. Also, theexchange of the AR request and the AR response can be realized by usingthe point-to-point ATM connections as shown in FIG. 8, or by using abroadcast channel.

Here, the address resolution procedure at the ARS 413 can be realized bythe search on the address space data (address mask) of the network.Namely, as will be described below, it suffices for the ARS 413 toanalyze the network address space in which the destination terminal ispresent, even for the destination terminal in the external network 416,and there is no need to resolve the VCI/VPI data for making a directaccess through the ATM connection to the destination terminal.

Thus, in this case, the datagram delivery is achieved by the dataexchange among the ARS 413, the CLSF 414, and the terminals 411 and 412as represented in FIG. 9, which shows a case in which a number ofdatagrams are taken at the CLSF 414 and then transmitted to the terminal412 collectively. Here, however, it is also possible to use the pipelinetype datagram transmission if desired.

2. Case Using VPI Routing

In this case, each network element of the sub-network 410 is assignedwith VPI as indicated in FIG. 10. Here, each terminal/server isequivalent to have a multipoint-to-point ATM connection from all UNI(User Network Interface) points within the sub-network 410. In otherwords, when the cell attached with the VPI of the destination terminalis outputted to the network from one terminal, the cell is transmittedto the destination UNI point. For example, in order to transmit the cellto the ARS 413, the VPI to be attached to the cell is VPI₄₁₃ assigned tothe ARS 413.

In this case, the procedure similar to that shown in FIG. 5 describedabove can be realized by using the connections 46A, 46B, and 46C.Namely, the terminal 411 transmits the AR request cell with the VPI dataof the VPI₄₁₃ to the ARS 413. Here, the ARS 413 can recognize that thereceived cell is transmitted from the terminal 411 according to the VCIdata or the upper layer identifier. For example, when the VPI data ofthe VPI1₄₁₁ assigned to the terminal 411 is written in the VCI field,the ARS 413 can recognize that the cell from the terminal 411 by the ATMheader. Alternatively, the VCI data can be the identification number forpositively identifying the cell as the AR request cell for the diagramtransmission.

Then, the ARS 413 carried out the address resolution for the destinationterminal 412, and writes VPI=VPI₄₁₁ in the AR response cell and outputsthe AR response cell to the network. Just as in the ARS 413, theterminal 411 then recognizes that the received cell is the AR responsecell according to the VCI data or the upper layer header data. Here, theAR response cell at least contains VPI₄₁₂ which is the access addressdata for the terminal 412.

Then, the terminal 411 transmits the cell for the datagram transmissionto the terminal 412. Here, the terminal 412 can recognizes that thereceived cell is the cell for the datagram transmission from theterminal 411 according to an identification data which is ether the VCIdata or the upper layer header data, and this identification data may benotified from the ARS 411 to the terminal 411 as the data content of theAR response.

As a simplest method for identifying the transmission source and thecell type at the receiving side in the above procedure, the VCI fieldcan be given by the first 8 bits indicating the VPI data of thetransmission source and the remaining 8 bits indicating the cell type.

For the cell transmitted from the terminal 411 to the terminal 412, thecoding of the VCI field (16 bits) can be any one of the following.

(1) The first 8 bits for the access address of the own terminal (thesame number as the VPI number), and the remaining 8 bits for theidentification number indicating the connection-less communication.

(2) The first 8 bits for the access address of the own terminal, and theremaining 8 bits for the identification number of the own network. Here,there is a need for each terminal to obtain the VPI for theconnection-less (CL) communication separately from the VPI for theconnection-oriented (CO) communication, i.e., to use different VPIs forCL and CO.

(3) All 16 bits set at a time of the boot of the terminal. It is alsopossible for the receiving side terminal to assign them appropriately.

On the other hand, the procedure similar to that shown in FIG. 8described above can be realized by using the connections 46A, 46B, 46D,and 46E. Namely, the terminal 411 transmits the AR request cell with theVPI data of the VPI₄₁₃ to the ARS 413. Then, the ARS 413 carried out theaddress resolution for the destination terminal 412, and writesVPI=VPI₄₁₄ in the AR response cell and outputs the AR response cell tothe network. Here, the AR response cell at least contains VPI₄₁₄ whichis the access address data for the CLSF 414. Also, the CLSF 414 canrecognizes that the received cell is the cell for the datagramtransmission from the terminal 411 according to an identification datawhich is either the VCI data or the upper layer header data, and thisidentification data may be notified from the ARS 411 to the terminal 411as the data content of the AR response.

The CLSF 414 then analyzes the address data of the received cell, andattaches the VPI=VPI₄₁₂ for transmitting the datagram cell to theterminal 412 and outputs the cell to the network. Then, the cell istransmitted to the terminal 412 through the ATM connection 46E.

In this case, any of the following five schemes can be adopted.

(1) The resolution of the access address for the destination terminal iscarried out by the CLSF 414. Here, the CLSF 414 analyzes the address ofthe destination terminal according to the network layer address (oraddress of the other layer if desired) written in the received datagram.At this point, the datagram is re-assembled at the CLSF 414 once.

(2) The resolution of the access address for the destination terminal iscarried out by the CLSF 414. Here, the CLSF 414 analyzes the address ofthe destination terminal according to the network layer address (oraddress of the other layer if desired) written in the received datagram.At this point, the datagram is not re-assembled at the CLSF 414 once,and the cell is relayed by the pipeline processing. In other words, theaccess address of the destination terminal is analyzed by analyzing theaddress data written in the first cell of the datagram, and thesubsequent cells are relayed to the designation terminal by rewritingthe VPI/VCI according to the VCI data of the first cell. Here, it isnecessary for the CLSF 414 to assign different VCI for each datagram.

(3) The resolution of the access address for the destination terminal iscarried out by the transmission terminal, and written into the payloadsection of the first cell. The CLSF 414 receiving the first cell thenreads the payload section of the first cell, and transmits the cell tothe destination terminal accordingly. At this point, the datagram isre-assembled at the CLSF 414 once.

(4) The resolution of the access address for the destination terminal iscarried out by the transmission terminal, and written into the payloadsection of the first cell. The CLSF 414 receiving the first cell thenreads the payload section of the first cell, and transmits the cell tothe destination terminal accordingly. At this point, the datagram is notre-assembled at the CLSF 414 once, and the cell is relayed by thepipeline processing. In other words, the access address of thedestination terminal is analyzed by analyzing the address data writtenin the first cell of the datagram, and the subsequent cells are relayedto the destination terminal by rewriting the VPI/VCI according to theVCI data of the first cell. Here, i tis necessary for the CLSF 414 toassign different VCI for each datagram.

(5) The resolution of the access address for the destination terminal iscarried out by the transmission terminal, and transmitted to the CLSF414 by utilizing the VCI field. In other words, the VCI is given by thefirst 8 bits for the access address of the destination terminal and theremaining 8 bits for the access address of the transmission sourceterminal. The CLSF 414 relays the cell to the destination terminal bycopying the destination terminal address in the VCI field. Here, therelaying can be done by ether re-assembling the datagram once, or by thepipeline type cell relaying.

DATAGRAM DELIVERY TO EXTERNAL NETWORK IN HIERARCHICAL NETWORK

Next, various schemes for the datagram delivery to the destinationterminal in the external network will be described.

1. General ATM network: Scheme I

In this case, an exemplary hierarchical network architecture is as sownin FIG. 11 which comprises networks 471 to 475 with the inter-networkingprovided by the IWUs 476 to 479, including a public network 475connected with the network 471 through an IWU 479. Here, each of theIWUs 476 to 479 can realize the relaying of the ATM cells withoutterminating the ATM connection, by heading a function to convert theVCI/VPI of the received cell into VCI/VPI assigned to the correspondingATM connection in the neighboring network.

FIG. 12 shows the setting of the ATM connections related to the addressresolution in which the networks 471 to 474 are also equipped with theARSs 481 to 484, respectively, each for managing the address data of theterminals or the network itself for at least one of the networks 471 to474 to which each of which belongs, where the ARS 481 is connected withthe ARSs 482 to 484 through the ATM connections 485 to 487.

FIG. 13 shows the setting of the ATM connections 496, 497, 498, and 49Brequired for transmitting the datagrams from the network 472 to theother networks 471, 472, 474, and 475, respectively, where the ATMconnections 486, 487, and 498 are single direction ATM connections fromthe IWU 476 of the network 472 to the CLSFs 491, 493, and 494,respectively, while the ATM connection 49B is an ATM connection to aCLSF (not shown) of the public network 475 which is usually set to bebidirectional. Although not shown in FIG. 13, the similar bidirectionalATM connection from each of the other networks 471, 473, and 474 to thepublic network 475 are also provided. Here, any of the CLSFs and theARSs can be located at positions of the corresponding IWUs if desired.Also, each corresponding CLSF and ARS can be located at the identicalposition.

For the connection-less communication cell to be transmitted from thepublic network 475 to any of the networks 471 to 474, there is provideda server (not shown) for terminating the connection-less communication(ATM connection) to the public network 475 once within the network 471,such that from the public network 475 side, this server is defied as theaccess point for the connection-less communication. This serverterminates the ATM connection and then transmits the datagram to thedestination terminal. Here, the datagram transmission from this serveris the same as the datagram transmission from each terminal to the otherterminal.

In this case, the terminal protocol, i.e., the procedure fortransmitting the datagram of the connection-less communication from oneterminal to the destination terminal is as follows.

(1) The terminal makes the AR request. This can be done either always oronly in a case the address resolution cannot be succeeded by theterminal itself such as a case in which a suitable entry is present inthe AR table.

(2) The terminal obtains the VCI/VPI data, which is the identifier ofthe ATM connection, provided from the ARS in order to make access to thedestination terminal.

(3) The terminal attaches the obtained VCI/VPI to the cell and outputsthe cell to the network, so as to perform the datagram transmission.

Here, at a time of the datagram transmission, there is no need for theterminal to carry out the procedure for setting up a particularconnection defined in the ATM network.

As for the protocol for the ARS in this case, the following two typesare available.

(i) A Backbone ARS

In this case, the star-shaped ATM connections (bidirectionalcommunication channels) have been established from the ARS 481 in thenetwork 471 to the ARSs 482 to 484 in the networks 472 to 474,respectively. For example, in order to obtain the VCI/VPI fortransmitting the datagram from the terminal 47A to the terminal 47D, theterminal 47A transmits the AR request cell having the address data ofthe terminal 47D to the ARS 482 of its own network 742. The ARS 482which received this AR request cell then recognizes that the address ofthe destination terminal written in the received cell does not belongsto its own network 472, and carries out the relaying of the AR requestcell through the already set up ATM connection 485 to the ARS 481 in thenetwork 471. Alternatively, it is also possible for the ARS 482 to cachethe network address data of the external network previously obtained andthe VCI/VPI data for transmitting the cell to the corresponding CLSFthrough the ATM connection 485 in advance.

The ARS 481 analyzes the VCI/VPI data for transmitting the datagram tothe CLSF in the network 474 to which the datagram is to be transmittedaccording to the address data of the destination terminal 47D written inthe received cell, and transmits the obtained VCI/VPI data to the ARS482. The ARS 482 then returns the AR response indicating the VCI/VPIdata to be used at the terminal 47A as the VCI/VPI data receives fromthe ARS 481.

Here, the address resolution at the ARS 481 is carried out as follows.Namely, the ARS 481 has the address data (address space data such asnet/ID) of the terminals contained in its own network 471 and theaddress space data for the sub-networks 472 to 474. Then, the ARS 481analyzes the transmission target network by comparing the addresswritten in the received AR request cell with the address space data foreach sub-network. Here, at a time of this comparison, it is notnecessary for the ARS 481 to analyze the host address of the destinationaddress, and it suffices to analyze only up to the network address.

As for the method for identifying the datagram direction to the publicnetwork 475, the following two methods are available.

(a) The address data written in the AR request cell indicates whether itis the diagram explicitly direction to the public network 475, or it isthe datagram not directed to the public network 475. In other words,this is a case in which the terminal knows whether it is the datagramdirected to the public network or not at the time of the AR request. Inthis case, the terminal 472 transmits the AR request cell in a form bywhich the ARS 481 can explicitly recognize whether it is the datagramdirected to the public network 475 or not. In a case it is the addressdata not coinciding with the addresses directed to the public network475, and when that address is not present in the address entries in theARS 481, the data indicating the absence of the requested address istransmitted to the ARS 482.

(b) When the address written in the received AR request cell cannot befound in the address entries in the ARS 481, this address is judged asthat which belongs to the public network 475.

In this manner, the ARS 481 possesses the addresses and the addressspace data of the terminals belonging to the network 471 and thesub-networks 472, 473, and 474, so as to carry out the addressresolution.

The address space views of the neighboring sub-networks from theperspectives of ARSs 481 and 482 in this case are shown in FIG. 14 andFIG. 15, respectively. As shown, from the ARS 428, the other networksare collectively appearing as the network 511 connected at the IWU 476and not as a plurality of sub-networks. The address space of eachsub-network can be resolved only after the data exchange with the ARS481 has taken place.

The address data (ATM layer address data) received by the ARS 482 fromthe ARS 481 is the data indicating the identifier (normally VCI/VPI, butmay contain the identification data for the upper layer) of the ATMconnection to the CLSF in the target sub-network from the IWU 476. Forexample, at a time of the datagram transmission from the terminal 47A tothe terminal 47D, the identifiers of the ATM connections 495 and 497 foraccessing the CLSF 494 are notified as the AR response, whereas at atime of the datagram transmission from the terminal 47A to the terminal47F, the identifiers of the ATM connections 495 and 496 for accessingthe CLSF 491 are notified as the AR response. Here, the ARS 482 notifiesthe VCI/VPI data for correctly relaying the ATM connection at the IWU476 to the terminal 47A, according to the VCI/VPI data received from theARS 481. The VCI/VPI data notified to the terminal 47A is then rewritteninto the other VCI/VPI at the IWU 476.

(ii) A Front End ARS

In this case, the star-shaped ATM connections as shown in FIG. 12described above is formed from the ARS 481 in the network 471 to theARSs 482 to 484 in the networks 472 to 474, respectively, or the meshedATM connections as shown in FIG. 16 is formed by the ARSs 481 to 484.Each ARS obtains the address space data and the ATM connection data(VCI/VPI) for the external sub-networks viewed from its own sub-networkusing the ATM connections defined as shown in FIG. 12 or FIG. 16. Here,FIG. 12 is in a form in which the ARS 481 functions as a master ARS,while FIG. 16 is in a distributed form in which each ARS operatesindependently. In a case the number of hierarchical levels in thenetwork is at most three, a case of FIG. 12 to make the backbone networkas the master is more appropriate.

On the other hand, in a case there is not limit to the number ofhierarchical levels in the network, which one of FIG. 12 and FIG. 16 isto be selected depending on the form of the network, management state,and a number of sub-networks provided in the network. For example, inorder to obtain the VCI/VPI for transmitting the datagram from theterminal 47A to the terminal 47D, the terminal 47A transmits the ARrequest cell having the address data of the terminal 47D to the ARS 482of its own network 472. The ARS 482 which received this AR request cellthen recognizes that the address of the destination terminal written inthe received cell belongs to the network 474, and transmits the VCI/VPIdata for transmitting the datagram to the CLSF in the network 474 towhich the datagram is to be transmitted according to the address data ofthe destination terminal 474 to the terminal 47A as the AR response.

Here, the address resolution at the ARS 482 is carried out as follows.Namely, the ARS 482 has the address data (address space data) of theterminals contained in its own network 472 and the address space datafor the sub-networks 471, 473, and 474. Then, the ARS 482 analyzes thetransmission target network by comparing the address written in thereceived AR request cell with the address space data for eachsub-network. Here, at a time of this comparison, it is not necessary forthe ARS 482 to analyze the host address of the destination address, andit suffices to analyze only up to the network address.

As for the method for identifying the datagram directed to the publicnetwork 475, the following two methods are available.

(a) The address data written in the AR request cell indicates whether itis the datagram explicitly directed to the public network 475, or it isthe diagram not directed to the public network 475. In other words, thisis a case in which the terminal knows whether it is the datagramdirected to the public network or not at the time of the AR request. Inthis case, the terminal 472 transmits the AR request cell in a form bywhich the ARS 481 can explicitly recognize whether it is the datagramdirected to the public network 475 or not. In a case it is the addressdata not coinciding with the addresses directed to the public network475, and when that address is not present in the address entries in theARS 482, it is judged that the requested address is absent.

(b) When the address written in the received AR request cell cannot befound in the address entries in the ARS 482, this address is judged asthat which belongs to the public network 475.

In this manner, the ARS 482 possesses the addresses and the addressspace data of the terminals belonging to the network 472 and thesub-networks 471, 473, and 474, so as to carry out the addressresolution.

The address space views of the neighboring sub-networks from theperspectives of ARSs 482 and 481 in this case are shown in FIG. 17 andFIG. 18, respectively. As shown, from either one of the ARS 481 and theARS 482, the entire address space of each sub-network can be resolved.

The address data (ATM layer) received by the ARS 482 from the other ARSis the data indicating the identifier (normally VCI/VPI, but may containthe identification data for the upper layer) of the ATM connection tothe CLSF in the target sub-network from the IWU 476. For example, at atime of the datagram transmission from the terminal 47A to the terminal47D, the identifiers of the ATM connections 495 and 497 for making anaccess to the CLSF 494 are notified as the AR response, whereas at atime of the datagram transmission from the terminal 47A to the terminal47F, the identifiers of the ATM connections 495 and 496 for making anaccess to the CLSF 491 are notified as the AR response. Here, the ARS482 notifies the VCI/VPI data for correctly relaying the ATM connectionat the IWU 476 to the terminal 47A, according to the VCI/VPI datareceived from the other ARS. The VCI/VPI data notified to the terminal47A is then rewritten into the other VCI/VPI at the IWU 476.

Among ARSs, not only the address space data exchange protocol for eachsub-network, but also the routing protocol concerning the datagramtransmission (connection-less communication) among the sub-networks isalso operated. More specifically, this routing protocol carries out themanagement of the ATM connection setting between the IWU and the CLSF asshown in FIG. 13 described above. Here, the individual ATM connection isseparated at the IWU (i.e., closed within the sub-network), and the ATMconnection routing control and the ATM connection management (such asVCI/VPI management) are made by the other ATM connection server processand the routing server process, and the ARS exchanges the controlmessages with these servers as well as the IWU to carry out themanagement of the ATM connections necessary for the connection-lesscommunication.

An example of a VCI/VPI rewriting table to be possessed by the IWU 476is shown in FIG. 19. Here, there is not need to use the different outputVCI/VPI to be attached in transmitting the cell to the same CLSF. It isalso possible to combine the VCI/VPI data with the identification datain the upper layer unit such that it becomes possible for thedestination CLSF to recognize the datagram to which the received cellbelongs. Similarly, there is no need to use the different input VCI/VPIattached to the cells transmitted from the same terminal.

There is also a case requiring the use of the ATM connection havingdifferent route from the normally used ATM connection due to thecongestion or the obstruction in the sub-network. In such a case, therouting control process for carrying out the management control of theVCI/VPI to be set up in the table of each switch as well as the routingtable data (where the control process for carrying out the addressmanagement can be the other process) carries out the management controlsuch that the VCI/VPI the same VCI/VPI to be shows to the UNI can bemaintained even when the route within the sub-network is changed.

Next, when the access point of the various server such as CLSF is moved,it suffices to reboot it such that the connection identifiers (VCI/VPI)visible from each access point becomes the same. Also, at a time of thereboot, with respect to the related access point, the request messagefor discarding the data for the old access from the AR table istransmitted along with the new access data if necessary. In the altercase, there is a need for the IWU to update the entry data in the tableof FIG. 19, either according to the data within the transmitted messageor by executing the protocol for obtaining the new data.

Next, the routing to the destination terminal in this case will bedescribed. The final delivery of the datagram to each terminal iscarried out by each CLSF only for its own network. For example, the CLSF491 carries out the datagram delivery to the terminals belonging to thenetwork 471 but not for the networks 472, 473, 474, and 475. Similarly,the CLSF 494 carries out the datagram delivery to the terminals withinthe network 494 alone. When the network address possessed by thedatagram received at each CLSF is not present in the address entriespossessed by that CLSF, or when the address of the received datagram isnot elements of the network address space of the network, it is judgedthat that datagram has been transmitted incorrectly. The treatment ofthe erroneously delivered datagram will not be discussed here.

Thus, it suffices for each CLSF to possess only the address data of theterminals of the network to which that CLSF itself belongs. When theaddress of the received datagram is present in its own network, theappropriate ATM connection is selected and the relaying of the datagramis carried out.

An exemplary protocol processing in a case of transmitting the datagramfrom the terminal 47A to the terminal 47D is shown in FIG. 20. Namely,the ATM connection is terminated at the CLSF 494 once, i.e., theprotocol of the OSI layer 3 is terminated at the CLSF 494. The OSI layer3 protocol processing is carried out at the CLSF 494, and the data unitis transmitted to the terminal 47D using the ATM connection. In thismanner, at a time of the datagram transmission to the terminal otherthan those of its own sub-network, the end-to-end datagram delivery canbe realized with only one ATM connection termination.

Now, more concrete example of this case will be described in detail.Namely, the exemplary case of transmitting the datagram from theterminal 47A to the terminal 47D will be illustrated for two situationsshown in FIG. 21 corresponding to the management views of FIGS. 14 and15, and FIG. 22 corresponding to the management views of FIGS. 17 and18.

In this case, the address resolution is carried out as follows.

In transmitting the datagram from the terminal 47A to the terminal 47D,when the terminal 47A does not possess the ATM layer address data fortransmitting the datagram to the terminal 47D, the terminal 47Atransmits the AR request cell having the address data of the terminal47D to the ARS 482 through the ATM connection 571 (581). Here, the ATMconnection 571 (581) can be realized by either the point-to-pointconnection or the broadcast connection. The ARS 482 which received theAR request cell recognizes that the address requested by the AR requestcell does not belongs to its own network 472, so that in the situationof FIG. 21, the ARS 482 transmits the AR request cell to the ARS 482through the ATM connection 572. Here, the procedure for the ARS 482 toreceive the AR request cell and then transmit the AR request cell to theARS 481 can be any of the following.

(1) In a case the ARS is responsible for responding to all the ARrequest cells generated within its network, the ARS carries out thescreening of the AR request cells first. Namely, whether the address inthe received cell is the element of the address space of its own networkor not is checked by using the network mask, etc. If the address belongsto its own network, the address table is looked up and the ATM layeraddress data for the appropriate terminal is returned. On the otherhand, when the address does not belongs to its own network, the ARrequest cell is transmitted to the prescribed superior ARS which is theARS 481 in the example of FIG. 21.

(2) In a case the ARS makes the response only to the request fortransmitting the datagram to the terminal belonging to the sub-networkother than its own network, and the response to the AR request for theterminal within its own network is returned by the terminal to which thedatagram is to be transmitted itself, the result differs for a case ofusing the broadcast channel and a case of using the point-to-pointconnection at a time of the address resolution. In a case of using thebroadcast channel, after the screening of the address is carried out,the AR request cell is taken and transmitted to the superior ARS. On theother hand, in a case of using the point-to-point connection, the ARrequest cell is transmitted to the superior ARS unconditionally inprinciple.

In the example of FIG. 22, the ARS 481 which received the AR requestcell checks if the address in the AR request cell is contained in theaddresses and the address space contained in its own network. In a casethe corresponding address is not present in the address entries, it isjudged as the address directed to the public network. In this example,the destination terminal is the terminal 47D, so that the ARS 471recognizes that the address in the AR request cell is present in theaddress space of its own network 474 and therefore the address spacedata possessed by the network 474 can b used. The ARS 481 which analyzedthe address of the terminal 47D returns the response indicating theVCI/VPI data of the ATM connection from the IWU 476 to the CLSF 494(VCI/VPI data viewed from the IWU 476) to the ARS 482.

The ARS 482 then returns the VCI/VPI data which is the identificationdata of the ATM connection to be relayed to the ATM connection connectedto the CLSF 494 at the IWU 476 to the terminal 47A. By using theVCI/VPI, the terminal 47A can deliver the datagram directly to the CLSF494.

In the example of FIG. 22 in which the ARS 482 can make the addressresolution by itself, with respect to the AR request given through theATM connection 581, the AR response can be directly transmitted to theterminal 47A through the ATM connection 582.

Now, in this case, the datagram delivery is carried out as follows.

The terminal 47A transmits the cell having the datagram information tothe CLSF 494 through the ATM connection 575 (583). The IWUs 476 and 478carries out the relaying of the ATM connection by rewriting the VCI/VPIdata of the received cell. This processing is carried out as the ATMlayer processing without raising it to the upper layer, so that the ATMconnection 575 (583) passing through the IWUs 476 and 478 can beregarded as one ATM connection without the ATM terminal point.

At the CLSF 494 which is the terminal point of the ATM connection 575(583), the network layer processing is carried out. The CLSF 494 carriesout the analysis of the network address, and the datagram is relayed tothe ATM connection 576 (584) and transmitted to the terminal 47D.

In this manner, there are only two ATM connections frOm the terminal 47Ato the CLSF 494, and from the CLSF 494 to the terminal 47D, so that theyare terminated only once.

2. Case Using VPI Routing: Scheme I

In this case, the ATM layer address assignment method will be describedfirst.

In a case of carrying out the VPI routing, the VPI field (VPI-F)indicates the destination UNI. There is a need to define the codingscheme of the VCI field as follows. Here, the coding of the VCI field isrelated to the cell transmission concerning the connection-lesscommunication, and it is not necessary to use this coding for the otherapplications such as the connection-oriented connection. In other words,in general, the coding scheme of the VCI field can be carried out by thenegotiation between the transmitting terminal and the receivingterminal. Here, 16 bits of the VCI field are defined to form two 8 bitssub-fields referred hereafter as VCI-F1 and VCI-F2.

In this case, the ATM connections are set up such that the setting ofthe ATM connections related to the address resolution are as shown inFIGS. 23 to 25. Here, each of the ARS 481 to 484 is managing the addressdata for the terminals or the network contained in at least one of thenetworks 471 to 474 to which it belongs.

FIG. 23 is a case of managing the address data of each sub-network withthe ARS 481 playing the leading role (of a root ARS). The VCI/VPI dataare rewritten by the IWU and the relaying of the ATM connections iscarried out. For example, the ATM connection from the ARS 482 to the ARS481 is formed by the connections 591 and 592. The connection 591 has theVPI-F having VPI₄₇₆₋₂ which is the access address (VPI) of the IWU 476for the network 472, and the VCI-F1 having VPI₄₈₂ which is the accessaddress of the ARS 482. As for the VCI-F2, it can take an arbitraryvalue in principle, but it is coded such that the IWU 476 can identifythe received cell as that which is to be relayed to the connection 592from the data content of the VCI-F2. On the other hand, the connection592 has the VPI-F having VPI₄₈₁ which is the access address of the ARS481, and the VCI-F1 having VPI₄₇₆₋₁ which is the access address of theIWU 476 for the network 471. Here, in both of these connections 591 and592, the VCI-F2 can take an arbitrary value in principle.

The IWU 476 analyzes the VCI/VPI data to be written into the cell of theconnection 592 from a set of data VCI-F1 and VCI-F2 of the receivedcell. Therefore, the IWU 476 has the 16 bits VCI field as the tableentry, and as a result, has the function to analyze the VCI/VPI data.The VPI-F of the cell of the connection 592 is analyzed by thecombination of VCI-F1 and VCI-F2. To the VCI-F1, VPI₄₇₆₋₁ is written,while the VCI-F2 is coded as the identifier of the ATM connectionbetween the ARS 482 to the ARS 481. The value of the VCI-F2 isdetermined at a time of setting the ATM connections (connections 591 and592). The assignment of the VCI-F2 can be made by the process formanaging the VCI-F2 within the sub-networks (networks 471 and 472), orby the process for managing the value of the VCI-F2 at the terminals(ARS 481 and IWU 476).

Next, FIGS. 24 and 25 show a case of managing the address data of eachsub-network by each ARS independently as in a case of FIG. 16 describedabove.

For example, the ATM connection 617 from the ARS 482 to the ARS 484 isformed by the connection 6021, 6041, and 6061. The connection 6021 hasthe VPI-F having VPI₄₇₆₋₂ which is the access address of the IWU 476 forthe network 472, and the VCI-F1 having VPI₄₈₂ which is the accessaddress of the ARS 482. The VCI-F2 is an identification number allocatedto this connection 6021 such that this connection 6021 can be identifiedby the combination of the VCI-F1 and the VCI-F2. The connection 6061 hasthe VPI-F having VPI₄₇₈₋₁ which is the access address of the IWU 478 forthe network 471, and the VCI-F1 having VPI₄₇₆₋₁ which is the accessaddress of the IWU 476 for the network 471. The VCI-F2 is a value set ata time of setting this connection 6061. The connection 6041 has theVPI-F having VPI₄₈₄ which is the access address of the ARS 484, and theVCI-F1 having VPI₄₇₈₋₁ which is the access address of the IWU 478 forthe network 471. The VCI-F2 is a value set at a time of setting thisconnection 6041.

For example, the IWU 478 analyzes the VCI/VPI data to be written intothe cell of the connection 6041 frOm a set of data VCI-F1 and VCI-F2 ofthe received cell. Therefore, the IWU 478 has the 16 bits VCI field asthe table entry, and as a result, has the function to analyze theVCI/VPI data. The VPI-F of the cell of the connection 6041 is analyzedby the combination of VCI-F1 and VCI-F2. To the VCI-F1, VPI₄₇₈₋₁ iswritten, while the VCI-F2 is coded as the identifier of the ATMconnection between the ARS 482 to the ARS 481 by the analysis of the VCIfield data. The value of the VCI-F2 is determined at a time of settingthe ATM connections (connections 6021, 6041, and 6061). The assignmentof the VCI-F2 can be made by the process for managing the VCI-F2 withinthe sub-networks (networks 471 472, and 474), or by the process formanaging the value of the VCI-F2 at the terminals (ARS 481, IWU 476, andARS 484).

By the setting scheme of the VCI/VPI field as in the above, even whenthe ARS has transmitted the data over a plurality of cells to the otherARS, the re-assembling of the data to the receiving side can be donewithout any problem. Here, for the data identification such as whichterminal has the data transmitted from or whether the data is related tothe address resolution protocol, there is a need to use the upper layeridentification field.

FIG. 28 shows the setting of the ATM connections required fortransmitting the datagram from the terminal 47A to the terminal 47F, thepublic network 475, and the terminal 47D. Here, the datagramtransmission can be carried out in either one of the following twoschemes.

(1) Scheme 1

First, the datagram transmission from the terminal 47A to the terminal47F will be described.

In this case, two ATM connections are required. One is formed by theconnections 621 and 624 between the terminal 47A and the CLSF 491, whilethe other is formed by the connection 625 between the CLSF 491 and theterminal 47F. The terminal 47A transmits to the IWU 476 the cell withthe VPI-F having VPI₄₇₆₋₂ which is the access address of the IWU 476 forthe network 472, the VCI-F1 having the VPI_(47A) which is the accessaddress of the terminal 47A, and the VCI-F2 having a value set at a timeof setting the connection 621. The IWU 476 analyzes the VCI of thereceived cell, as well as the corresponding VPI-F and the VCI-F2. Then,to the VCI-F1, VPI₄₇₆₋₁ which is the access address of the IWU 476 forthe network 471 is written. Here, the VCI-F2 is a value which isdetermined at a time of setting the connection 624, which can be writtenby copying the value of the VCI-F1 of the cell received by the IWU 476,i.e., the VPI_(47A) which is the access address of the terminal 47A.

The datagram arrived at the CLSF 491 is cell re-assembled once, and theATM connection is terminated. The CLSF 491 analyzes the upper layeraddress data and recognizes that the destination of the datagram as theterminal 47F. Then, the CLSF 491 generates the cell with the VPI-Fhaving VPI_(47F) which is the access address of the terminal 47F, theVCI-F1 having the VPI₄₉₁ which is the access address of the CLSF 491,and the VCI-F2 having an identification number assigned to theconnection 625, and transmits the datagram to the terminal 47F. Here,the terminal 47F can uniquely identify the datagram to which the cellbelongs from the VCI-F1 and the VCI-F2. Namely, the datagram can bere-assembled at the terminal 47F.

Next, the datagram transmission from the terminal 47A to the publicnetwork 475 will be described.

In this case, only one ATM connection formed by the connections 622,626, and 627 between the terminal 47A and the public network 475 isrequired. The terminal 47A transmits to the IWU 476 the cell with theVPI-F having VPI₄₇₆₋₂ which is the access address of the IWU 476 for thenetwork 472, the VCI-F1 having the VIP_(47A) which is the access addressof the terminal 47A, and the VCI-F2 having a value set at a time ofsetting the connection 622. The IWU 476 analyzes the VCI of the receivedcell, as well as the corresponding VPI-F and the VCI-F2. Then, to theVCI-F1, VPI₄₇₆₋₁ which is the access address of the IWU 476 for thenetwork 471 is written. Here, the VCI-F2 is a value which is determinedat a time of setting the connection 626, which can be written by copyingthe value of the VCI-F1 of the cell received by the IWU 476, i.e., theVPI_(47A) which is the access address of the terminal 47A. The IWU 479writes the VCI/VPI assigned to the ATM connection 627 defined by thepublic network 475 according to the VCI data of the received cell, andtransmits the cell to the public network 475.

Finally, the datagram transmission from the terminal 47A to the terminal47D will be described.

In this case, two ATM connections are required. One is formed by theconnections 623, 628, and 629 between the terminal 47A and the CLSF 494,while the other is formed by the connection 62A between the CLSF 494 andthe terminal 47D. The terminal 47A transmits to the IWU 476 the cellwith the VPI-F having VPI₄₇₆₋₂ which is the access address of the IWU476 for the network 472, the VCI-F1 having the VPI_(47A) which is theaccess address of the terminal 47A, and the VCI-F2 having a value set ata time of setting the connection 623. The IWU 476 analyzes the VCI ofthe received cell, as well as the corresponding VPI-F and the VCI-F2.Then, to the VCI-F1, VPI₄₇₆₋₁ which is the access address of the IWU 476for the network 471 is written. Here, the VCI-F2 is a value which isdetermined at a time of setting the connection 628, which can be writtenby copying the value of the VCI-F1 of the cell received by the IWU 476,i.e., the VPI_(47A) which is the access address of the terminal 47A.

Next, the IWU 478 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. Here, there is a need to assign theVCI/VPI such that the CLSF 494 can identify the cell even when the cellsarrive from all the terminal (capable of transmitting datagram to theCLSF 494) within the network simultaneously.

The datagram arrived at the CLSF 494 is cell re-assembled once, and theATM connection is terminated. The CLSF 494 analyzes the upper layeraddress data and recognizes that the destination of the datagram as theterminal 47D. Then, the CLSF 494 generates the cell with the VPI-Fhaving VPI_(47D) which is the access address of the terminal 47D, theVCI-F1 having the VPI₄₉₄ which is the access address of the CLSF 494,and the VCI-F2 having an identification number assigned to theconnection 62A, and transmits the datagram to the terminal 47D. Here,the terminal 47D can uniquely identify the datagram to which the cellbelongs from the VCI-F1 and the VCI-F2. Namely, the datagram can bere-assembled at the terminal 47D.

(2) Scheme 2

In this scheme, each CLSF has obtained the VPI address, and all theconnections identified by the obtained VPIs are ATM connections to beused for the cell transmission related to the datagram transmission(connection-less communication). Namely, the CLSF server requires theATM connection (not connection-less one) in order for itself to carryout the communication with the other terminal/server. In other words,there is a need for the CLSF to obtain at least tow access addresses(VPIs) at a time of the boot.

Similarly, each IWU also obtains at least two access addresses (VPIs) ata time of the boot. One VPI is used as that related to the cell for theconnection-less communication. Namely, in a case of transmitting thecell for the datagram related to the connection-less communication tothe external network, each terminal outputs the cell to the network byattaching the VPI (for IWU) which is defined for the connection-lesscommunication.

At this point, the following coding scheme for the VCI field of the cellrelated to the connection-less communication is used. Here, 16 bits ofthe VCI field are defined to form two 8 bits sub-fields VCI-F1 andVCI-F2, but the positions of these sub-fields is not specified.

First, the cell to be transmitted within the external sub-network iscoded as follows. Namely, this coding scheme is the coding of the VCIfield for a case in which the cell is transmitted from the sub-networkof the transmitting terminal to the external sub-network via the IWU.The VIP-F1 has the identification number of the sub-network of thetransmitting terminal, while the VCI-F2 has the identification number ofthe transmitting terminal within the sub-network. For example, as theidentification number of the sub-network, it is possible to set theaccess address (VPI) of each IWU in the network 471 as theidentification address of the sub-network, in which case the network 471itself is coded appropriately. Also, as the identification number of theterminal, it is possible to set the access address (VPI) of the terminalin each sub-network. For example, the VCI field of the cell for theconnection-less communication which is outputted from the terminal 47Dand transmitted within the external sub-network, VPI-F1 can be VPI₄₇₈₋₁and VCI-F2 can be VPI_(47D).

Next, the cell to be transmitted within its own sub-network is coded asfollows. Namely, this coding scheme is the coding of the VCI field for acase in which the cell is transmitted to the IWU of the sub-network ofthe transmitting terminal. The VPI-F1 has the identification number ofthe sub-network of the destination terminal. while the VCI-F2 has theidentification number of the transmitting terminal within thesub-network. For example, the VCI-F2 field of the cell for theconnection-less communication which is outputted from the terminal 47Dand transmitted within its own sub-network can be VPI_(47D).

By the scheme as in the above, in the procedure for the addressresolution which precedes the datagram transmission, the terminal cantransmits the datagram (one or more cells) as long as the identificationnumber of the sub-network to which the destination terminal belongs.This datagram transmission will now be described in detail.

First, the datagram transmission from the terminal 47A to the terminal47F will be described.

In this case, the terminal 47A transits to the IWU 476 the cell with theVPI-F having VPI₄₇₆₋₂ which is the access address of the IWU 476 for thenetwork 472, the VCI-F1 having he identification number of the network471 (which can be the VPI₄₉₄ which is the access address of the CLSF494), and the VCI-F2 having VPI_(47A) which is the access address of theterminal 47A.

The IWU 476 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI₄₉₁ which is the access address of the CLSF 491, the VPI-F ofthe cell to be outputted can be obtained by copying the VCI-F1 of thereceived cell. Then, to the VCI-F1, VPI₄₇₆₋₁ which is the access addressof the IWU 476 for the network 471 is written. Here, the VCI-F2 can beset to be transparent. Namely, in a case the VCI-F1 of the cell to betransmitted from the terminal 47A has VPI₄₉₁, the relaying of the cellcan be realized by the procedure of (1) copying the VCI-F1 of thereceived cell to the VPI-F of the transmission cell, (2) copying theVCI-F2 of the received cell to the VCI-F2 of the transmission cell, and(3) writing VPI₄₇₆₋₁ to the VCI-F1 of the transmission cell.

The datagram arrived at the CLSF 491 is cell re-assembled once, and theATM connection is terminated. The CLSF 491 analyzes the upper layeraddress data and recognizes that the destination of the datagram as theterminal 47F. Then, the CLSF 491 generates the cell with the VPI-Fhaving VPI_(47F) which is the access address of the terminal 47F, andtransmits the datagram to the terminal 47F. Here, the CLSF 491 canuniquely identify the datagram to which the cell belongs from the VCI-F1and the VCI-F2. Namely, the datagram can be re-assembled at the CLSF491. Therefore, it is also possible to carry out the pipeline typerelaying of the datagram (cell).

Next, the datagram transmission from the terminal 47A to the publicnetwork 475 will be described.

In this case, the terminal 47A transits to the IWU 476 the cell with theVPI-F having VPI₄₇₆₋₂ which is the access address of the IWU 476 for thenetwork 472, the VCI-F1 having the identification number of the publicnetwork 475 (which can be the VPI₄₇₉ which is the access address of theIWU 479), and the VCI-F2 having VPI_(47A) which is the access address ofthe terminal 47A.

The IWU 476 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI₄₇₉ which is the access address of the IWU 479, the VPI-F ofthe cell to be outputted can be obtained by copying the VCI-F1 of thereceived cell. Then, to the VCI-F1, VPI₄₇₆₋₁ which is the access addressof the IWU 476 for the network 471 is written. Here, the VCI-F2 can beset to be transparent. Namely, in a case the VCI-F1 of the cell to betransmitted from the terminal 47A has VPI₄₇₉, the relaying of the cellcan be realized by the procedure of (1) copying the VCI-F1 of thereceived cell to the VPI-F of the transmission cell, (2) copying theVCI-F2 of the received cell to the VCI-F2 of the transmission cell, and(3) writing VPI₄₇₆₋₁ to the VCI-F1 of the transmission cell. The IWU 479writes the VCI/VPI assigned to the ATM connection 627 defined by thepublic network 475 according to the VCI data of the received cell, andtransmits the cell to the public network 475.

Finally, the datagram transmission from the terminal 47A to the terminal47D will be described.

In this case, the terminal 47A transits to the IWU 476 the cell with theVPI-F having VPI₄₇₆₋₂ which is the access address of the IWU 476 for thenetwork 472, the VCI-F1 having the identification number of the network474 (which can be the VIP₄₇₈₋₁ which is the access address of the IWU478 for the network 471), and the VCI-F2 having VPI_(47A) which is theaccess address of the terminal 47A.

The IWU 476 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI₄₇₈₋₁ which is the access address of the IWU 478 for thenetwork 471, the VPI-F of the cell to be outputted can be obtained bycopying the VCI-F1 of the received cell. Then, to the VCI-F1, VPI₄₇₆₋₁which is the access address of the IWU 476 for the network 471 iswritten. Here, the VCI-F2 can be set to be transparent. Namely, in acase the VCI-F1 of the cell to be transmitted from the terminal 47A hasVPI₄₇₈₋₁, the relaying of the cell can be realized by the procedure of(1) copying the VCI-R1 of the received cell to the VPI-F of thetransmission cell, (2) copying the VCI-F2 of the received cell to theVCI-F2 of the transmission cell, and (3) writing VPI₄₇₈₋₁ to the VCI-F1of the transmission cell.

The IWU 478 analyzes the VCI of the received cell, as well as thecorresponding VPI-F, VCI-F1, and the VCI-F2. The VCI field of the cellto be transmitted from the IWU 478 to the CLSF 494 can set the VCI fielddata of the received cell to be transparent. Namely, it is possible tomake the VCI-F1 having VPI₄₇₆₋₁ and the VCI-F2 having VPI₄₇₆. Here, theVPI-F is set to be VPI₄₉₄ which is the access address of the CLSF 494.

The datagram arrived at the CLSF 494 is cell re-assembled once, and theATM connection is terminated. The CLSF 494 analyzes the upper layeraddress data and recognizes that the destination of the datagram as theterminal 47D. Then, the CLSF 494 generates the cell with the VPI-Fhaving VPI_(47D) which is the access address of the terminal 47D, andtransmits the datagram to the terminal 47D. Here, the terminal 47D canuniquely identify the datagram to which the cell belongs from the VCI-F1and the VCI-F2. Namely, the datagram can be re-assembled at the terminal47D. More specifically, for example, the sub-network to which thetransmission source terminal belongs can be determined from the VCI-F1,and the IWU which contains that sub-network can be determined from theVCI-F2 as the identification number of that sub-network.

3. General ATM Network: Scheme II

In this case, an exemplary hierarchical network architecture is as shownin FIG. 27 which comprises networks 631 to 635 with the inter-networkingprovided by the IWUs 636 to 639, including a public network 635connected with the network 631 through an IWU 639. Here, each of theIWUs 636 to 639 can realize the relaying of the ATM cells withoutterminating the ATM connection, by having a function to convert theVCI/VPI of the received cell into VCI/VPI assigned to the correspondingATM connection in the neighboring network.

In this configuration of FIG. 27, the networks 631 to 634 are alsoequipped with the ARSs 63G, 63D, 63E, and 63F, respectively, each formanaging the address data of the terminals or the network itself for atleast one of the networks 631 to 634 to which each of which belongs. Inaddition, the network 631 to 634 are also equipped with the CLSF-O 63M,63N, 63P, and 63Q, and CLSF-I 63H, 63J, 63K, and 63L, respectively.Here, CLSF-O, CLSF-I, and ARS may be implemented integrally. Moreover,these CLSF-O, CLSF-I, and ARS may be provide don the IWU if desired.

FIGS. 28 and 29 show the two settings of the ATM connections related tothe address resolution, where the ARS 63G is connected with the ARSs63D, 63E, and 63F through the ATM connections.

FIG. 30 shows the setting of the ATM connections 641 to 646 required fortransmitting the datagram from the terminal 63A to the terminal 63C, thepublic network 635, and the terminal 63B, where the ATM connections 642,644, and 645 are single direction ATM connections from the CLSF-O 63M tothe CLSF-I 63L, the public network 635, and the CLSF-I 63J,respectively. Although not shown in FIG. 30, the similar ATM connectionsfrom each of the other sub-networks are also provided. In addition, theATM connections among the CLSF-I and the CLSF-O as shown in FIG. 31 arealso provided.

Here, in a case of the connection-less communication from the publicnetwork 635 to the terminal belonging to the defined networks 631 to634, the ATM connection related to the connection-less communicationfrom the public network 635 is terminated at a server for terminatingthis ATM connection for the connection-less communication and relayingthe datagram, which is provided in the network 631. The datagram istransmitted from this server to the destination terminal by the sameprocedure as described below for the datagram transmission from theterminal within the network to the terminal within the other network.

Now, in this case, the protocol at the terminal is as follows. Namely,when the terminal judges that the datagram is destined to the externalsub-network, the terminal transmits the datagram to the CLSF-O. Here,the ATM connection is assumed to be already set up between the terminaland the CLSF-O. Each terminal has the address space data (such asaddress masks) for the sub-network to which it belongs, so that it ispossible for each terminal to judge whether the destination terminal isthe terminal within its own network or the terminal of the externalsub-network.

In the network architecture shown in FIG. 27, the datagram delivery canbe realized by any of the following three schemes.

(1) In this scheme, the CLSF-O and the CLSF-I are located at the sameaccess point, and the star-shaped ATM connections are set up frombetween the CLSF and the terminals. The terminal transmits the cell tothe CLSF whenever the datagram transmission is to be carried out. Thedatagram delivery is entirely carried out by the CLSF. Namely, even thecommunication between the terminals within the same sub-network isrealized via the CLSF.

(2) The communication between the terminals within the same sub-networkis realized without the CLSF, while the communication with the terminalof the external network is realizes via the CLSF-O.

(3) The CLSF-O is used for the communication with the terminal of theexternal network, while the CLSF-I is used for the communication withthe terminal within the same network.

As for the protocol for the ABS in this case, the following two typesare available.

(i) A Backbone ARS

In this case, the star-shaped ATM connections (bidirectionalcommunication channels) are formed from the ARS 63G in the network 631to the ARSs 63D, 63E, and 63F in the networks 632 to 634, respectively,as shown in FIG. 28. For example, in order to utilize the ATM connectionset up between the CLSF-O 63M and the CLSF-I 63J so as to transmit thedatagram from the terminal 63A to the terminal 63B, there is a need forthe CLSF-O 63M to obtain the VCI/VPI data of this ATM connection. Inorder to obtain this VCI/VPI, the CLSF-O 63M transmits the AR requestcell having the address data of the terminal 47B to the ARS 63D of itsown network 632 which is the address data written in the receiveddatagram. The ARS 63D which received this AR request cell then analyzesthe address of the destination terminal written in the received cell,and when it is impossible to carry out the address resolution from thedata possessed by it, the ARS 63D relays the AR request cell to the ARS63G in the network 631 through the already set up ATM connection 651.

The ARS 63G analyzes the VCI/VPI data for transmitting the datagram tothe CLSF in the network 633 to which the datagram is to be transmittedaccording to the address data of the destination terminal 63B written inthe received cell, and transmits the obtained VCI/VPI data to the ARS63D. The ARS 63D then returns the AR response indicating the VCI/VPIdata to be used at the CLSF-O 63M as the VCI/VPI data received from theARS 3G.

Here, the address resolution at the ARS 63G is carried out as follows.Namely, the ARS 63G has the address data (address space data) of theterminals contained in its own network 631 and the address space datafor the sub-networks 632 to 634. Then, the ARS 63G analyzes thetransmission target network by comparing the address written in thereceived AR request cell with the address space data for eachsub-network.

As for the method for identifying the datagram directed to the publicnetwork 635, the following two methods are available.

(a) The address data written in the AR request cell indicates whether itis the datagram explicitly directed to the public network 635, or it isthe datagram not directed to the public network 635. In other words,this is a case in which the terminal knows whether it is the datagramdirected to the public network or not at the time of the AR request. Inthis case, the terminal 632 transmits the AR request cell in a form bywhich the ARS 63G can explicitly recognize whether it is the datagramdirected to the public network 635 or not. In a case it is the addressdata not coinciding with the addresses directed to the public network635, and when that address is not present in the address entries in theARS 63G, the data indicating the absence of the requested address istransmitted to the ARS 63D.

(b) When the address written in the received AR request cell cannot befound in the address entries in the ARS 63G, this address is judged asthat which belongs to the public network 635.

In this manner, the ARS 63G possesses the addresses and the addressspace data of the terminals belonging to the network 631 and thesub-networks 632, 633, and 634, so as to carry out the addressresolution. Here, it is not necessary for the ARS 63G to possess theaddress data up to the terminal level for the sub-network other than itsown sub-network, and it suffices to possess the address data only up tothe network address level.

The address data (ATM layer address data) received by the ARS 63D fromthe ARS 63G is the data indicating the identifier (normally VCI/VPI, butmay contain the identification data for the upper layer) of the ATMconnection to the CLSF-I in the target sub-network from the IWU 636. Forexample, at a time of the datagram transmission from the terminal 63A tothe terminal 63B, the identifier of the ATM connection 645 for making anaccess to the CLSF-I 63J is notified as the AR response, whereas at atime of the datagram transmission from the terminal 63A to the terminal63C, the identifier of the ATM connection 642 for making an access tothe CLSF-I 63L is notified as the AR response.

Here, the ARS 63D notifies the VCI/VPI data for correctly relaying theATM connection at the IWU 636 to the CLSF-O 63M, according to theVCI/VPI data received from the ARS 463G. The VCI/VPI data notified tothe CLSF-O 63M is then rewritten into the other VCI/VPI at the IWU 636.

(ii) A front end ARS

In this case, the star-shaped ATM connections as shown in FIG. 28described above is formed from the ARS 63G in the network 631 to theARSs 632 to 634 in the networks 632 to 634, respectively, or the meshedATM connections as shown in FIG. 29 is formed by the ARSs 63D to 63G.Each ARS obtains the address space data and the ATM connection data(VCI/VPI) for the external sub-networks viewed from its own sub-networkusing the ATM connections defined as shown in FIG. 28 or FIG. 29. Here,FIG. 28 is in a form in which the ARS 63G functions as a master ARS,while FIG. 29 is in a distributed form in which each ARS operatesindependently. In a case the number of hierarchical levels in thenetwork is at most three, a case of FIG. 28 to make the backbone networkas the master is more appropriate.

On the other hand, in a case there is no limit to the number ofhierarchical levels in the network, which one of FIG. 28 and FIG. 29 isto be selected depending on the form of the network, management state,and a number of sub-networks provided in the network. For example, inorder to obtain the VCI/VPI for transmitting the datagram from theCLSF-O 63M to the terminal 63B, the CLSF-O 63M transmits the AR requestcell having the address data of the terminal 63B to the ARS 63D of itsown network 632. The ARS 63D which received this AR request cell thenrecognizes that the address of the destination terminal written in thereceived cell belongs to the network 633, and transmits the VCI/VPI datafor transmitting the datagram to the CLSF in the network 633 to whichthe datagram is to be transmitted according to the address data of thedestination terminal 63B to the CLSF-O 63M as the AR response.

Here, the address resolution at the ARS 63D is carried out as follows.Namely, the ARS 63D has the address data (address space data) of theterminals contained in its own network 632 and the address space datafor the sub-networks 631, 633, and 634. Then, the ARS 63D analyzes thetransmission target network by comparing the address written in thereceived AR request cell with the address space data for eachsub-network.

As for the method for identifying the datagram directed to the publicnetwork 635, the following two methods are available.

(a) The address data written in the AR request cell indicates whether itis the datagram explicitly directed to the public network 635, or it isthe datagram not directed to the public network 635. In other words,this is a case in which the terminal knows whether it is the datagramdirected to the public network or not at the time of the AR request. Inthis case, the terminal 63D transmits the AR request cell in a form bywhich the ARS 63G can explicitly recognize whether it is the datagramdirected to the public network 635 or not. In a case it is the addressdata not coinciding with the addresses directed to the public network635, and when that address is not present in the address entries in theARS 63D, it is judged that the requested address is absent.

(b) When the address written in the received AR request cell cannot befound in the address entries in the ARS 63D, this address is judged asthat which belongs to the public network 635.

In this manner, the ARS 63D possesses the addresses and the addressspace data of the terminals belonging to the network 632 and thesub-networks 631, 633, and 634, so as to carry out the addressresolution. Here, it is not necessary for the ARS 63D to possess theaddress data up to the terminal level for the sub-network other than itsown sub-network, and it suffices to possess the address data only up tothe network address level.

The address data (ATM layer) received by the ARS 63D from the other ARSis the data indicating the identifier (normally VCI/VPI, but may containthe identification data for the upper layer) of the ATM connection tothe CLSF-I in the target sub-network from the IWU 636. For example, at atime of the datagram transmission from the terminal 63A to the terminal63B, the identifier of the ATM connection 645 for making an access tothe CLSF-I 63J is notified as the AR response, whereas at a time of thedatagram transmission from the terminal 63A to the terminal 63C, theidentifier of the ATM connection 642 for making an access to the CLSF-I63L is notified as the AR response. Here, the ARS 63D notifies theVCI/VPI data for correctly relaying the ATM connection at the IWU 636 tothe CLSF-O 63M, according to the VCI/VPI data received from the otherARS. The VCI/VPI data notified to the CLSF-O 63M is then rewritten intothe other VCI/VPI at the IWU 636.

Among ARSs, not only the address space data exchange protocol for eachsub-network, but also the routing protocol concerning the datagramtransmission (connection-less communication) among the sub-networks isalso operated. More specifically, this routing protocol carries out themanagement of the ATM connection setting as shown in FIG. 30 describedabove. Here, the individual ATM connection is separated at the IWU(i.e., closed within the sub-network), and the ATM connection routingcontrol and the ATM connection management (such as VCI/VPI management)are made by the other ATM connection server process and the routingserver process, and the ARS exchanges the control messages with theseservers as well as the IWU to carry out the management of the ATMconnections necessary for the connection-less communication.

Next, the routing to the destination terminal in this case will bedescribed. The final delivery of the datagram to each terminal iscarried out by each CLSF-I only for its own network. For example, theCLSF-I 63L carries out the datagram delivery to the terminals belongingto the network 631 but not for the networks 632, 633, 634, and 635.Similarly, the CLSF-I 63J carries out the datagram delivery to theterminals within the network 633 alone. When the network addresspossessed by the datagram received at each CLSF is not present in theaddress entries possessed by that CLSF, or when the address of thereceived datagram is not elements of the network address space of thenetwork, it is judged that that datagram has been transmittedincorrectly. The treatment of the erroneously delivered datagram willnot be discussed here.

Thus, it suffices for each CLSF to possess only the address data of theterminals of the network to which that CLSF itself belongs. When theaddress of the of the received datagram is present in its own network,the appropriate ATM connection is selected and the relaying of thedatagram is carried out.

An exemplary protocol processing is a case of transmitting the datagramfrom the terminal 63A to the terminal 63B is shown in FIG. 32. Namely,the ATM connection is terminated at the CLSF-O 63M and the CLSF-I 63J,i.e., the protocol of the OSI layer 3 is terminated at the CLSF-O 63Mand the CLSF-I 63J. In this manner, at a time of the datagramtransmission to the terminal other than those of its own sub-network,the end-to-end datagram delivery can be realized with only two ATMconnection termination.

Now, more concrete example of this case will be described in detail.Namely, the exemplary case of transmitting the datagram from theterminal 63A to the terminal 63B will be illustrated for a situationshown in FIG. 30.

In this case, the address resolution is carried out as follows.

In transmitting the datagram from the terminal 63A to the terminal 63B,when the terminal 63A recognizes that the terminal 63B belongs to theexternal sub-network, the terminal 63A transmits the datagram to theCLSF-O 63M. The CLSF-O 63M then analyzes the address data of thereceived datagram, and transmits the AR request cell having the addressdata of the terminal 63B to the ARS 63D when the CLSF-O 63M does nothave the ATM layer address data for transmitting the datagram to theterminal 63B (or to the CLSF-I 63J).

When the ARS 63D possesses the data enabling the address resolution, theAR request containing the VCI/VPI for transmitting the cell from theCLSF-O 63M to the CLSF-I 63J is directly transmitted. On the other hand,when the address resolution cannot be made at the ARS 63D, theappropriate ARS is accessed to carry out the address resolution. Whenthe address resolution is completed, the resulting AR response istransmitted to the CLSF-O 63M.

Now, in this case, the datagram delivery is carried out as follows.

The terminal 63A transmits the cell having the datagram information tothe CLSF-O 63M through the ATM connection 641. The CLSF-O 63M thenanalyzes the address data of the datagram, cell assembles the datagram,and transmits the cell to the CLSF-I 63J by using the ATM connection645. The IWUs 636 and 637 carries out the relaying of the ATM connectionby rewriting the VCI/VPI data of the received cell.

At the CLSF-I 63J which is the terminal point of the ATM connection 645,the network layer processing is carried out. The CLSF-I 63J carries outthe analysis of the network address, and the datagram is relayed to theATM connection 646 and transmitted to the terminal 63B.

In this manner, there are only three ATM connections from the terminal63A to the CLSF-O 63M, from CLSF-O 63M to the CLSF-I 63J, and from theCLSF-I 63J to the terminal 63B, so that they are terminated only twice.

4. Case using VPI routing: Scheme II

In this case, the manner of realizing the ATM connections set up amongthe ARSs is equivalent to that described above as 3. General ATMnetwork: Scheme II, so that it will not be repeated here.

With reference to FIG. 30 described above and FIG. 33, four availableschemes for the setting of the ATM connections required for transmittingthe datagram from the terminal 63A to the terminal 63C, the publicnetwork 635, and the terminal 63B will be described.

(1) Scheme 1

First, the datagram transmission from the terminal 63A to the terminal63C will be described.

In this case, three ATM connections are required. One is formed by theconnection 691 between the terminal 63A and the CLSF-O 63M, another isformed by the connections 692 and 693 between the CLSF-O 63M and theCLSF-I 63L, and still another is the connection 694 between the CLSF-I63L and the terminal 63C. The terminal 63A transmits to the IWU 636 thecell with the VPI-F having VPI_(63M) which is the access address of theCLSF-O 63M, and the VCI-F1 having VPI_(63A) which is the access addressof the terminal 63A. Then, the CLSF-O 63M transmits to the IWU 636 thecell with the VPI-F having VPI₆₃₆₋₂ which is the access address of theIWU 636 for the network 632, the VCI-F1 having the VPI_(63M) which isthe access address of the CLSF-O 63M, and the VCI-F2 having a value setat a time of setting the connection 692. The IWU 636 analyzes the VCI ofthe received cell, as well as the corresponding VPI-F and the VCI-F2.Then, to the VCI-F1, VPI₆₃₆₋₁ which is the access address of the IWU 636for the network 631 is written. Here, the VCI-F2 is a value which isdetermined at a time of setting the connection 693, which can be writtenby copying the value of the VCI-F1 of the cell received by the IWU 636,i.e., the VPI_(63M) which is the access address of the CLSF-O 63M.

The datagram arrived at the CLSF-I 63L is cell re-assembled once, andthe ATM connection is terminated. The CLSF-I 63L analyzes the upperlayer address data and recognizes that the destination of the datagramas the terminal 63C. Then, the CLSF-I 63L generates the cell with theVPI-F having VPI_(63C) which is the access address of the terminal 63C,the VCI-F1 having the VPI_(63L) which is the access address of theCLSF-I 63L, and the VCI-F2 having an identification number assigned tothe connection 694, and transmits the datagram to the terminal 63C.Here, the terminal 63C can uniquely identify the datagram to which thecell belongs from the VCI-F1 and the VCI-F2. Namely, the datagram can bere-assembled at the terminal 63C.

Next, the datagram transmission from the terminal 63A to the publicnetwork 635 will be described.

In this case, two ATM connections is required. One is formed by theconnection 691 between the terminal 63A and the CLSF-O 63M, while theother is formed by the connections 692, 695, and 696 between theterminal CLSF-O 63M and the public network 635. The terminal 63Atransmits the cell with the VPI-F having VPI_(63M) which is the accessaddress of the CLSF-O 63M. Then, the CLSF-O 63M transmits to the IWU 636the cell with the VPI-F having VPI₆₃₆₋₂ which is the access address ofthe IWU 636 for the network 632, the VCI-F1 having the VPI_(63M) whichis the access address of the CLSF-O 63M, and the VCI-F2 having a valueset at a time of setting the connection 692. The IWU 636 analyzes theVCI of the received cell, as well as the corresponding VPI-F and theVCI-F2. Then, to the VCI-F1, VPI₆₃₆₋₁ which is the access address of theIWU 636 for the network 631 is written. Here, the VCI-F2 is a valuewhich is determined at a time of setting the connection 695, which canbe written by copying the value of the VCI-F1 of the cell received bythe IWU 636, i.e., the VPI_(63M) which is the access address of theCLSF-O 63M. The IWU 639 writes the VCI/VPI assigned to the ATMconnection 696 defined by the public network 635 according to the VCIdata of the received cell, and transmits the cell to the public network635.

Finally, the datagram transmission from the terminal 63A to the terminal63B will be described.

In this case, three ATM connections are required. One is formed by theconnection 691 between the terminal 63A and the CLSF-O 63M, another isformed by the connections 692, 697, and 698 between the CLSF-O 63M andthe CLSF-I 63J, and still another is formed by the connection 699between the CLSF-I 63J and the terminal 63B. The terminal 63A transmitsthe cell with the VPI-F having VPI_(63M) which is the access address ofthe CLSF-O 63M. Then, the CLSF-O 63M transmits to the IWU 636 the cellwith the VPI-F having VPI₆₃₆₋₂ which is the access address of the IWU636 for the network 632, the VCI-F1 having the VPI_(63M) which is theaccess address of the CLSF-O 63M, and the VCI-F2 having a value set at atime of setting the connection 692. The IWU 636 analyzes the VCI of thereceived cell, as well as the corresponding VPI-F and the VCI-F2. Then,to the VCI-F1, VPI₆₃₆₋₁ which is the access address of the IWU 636 forthe network 631 is written. Here, the VCI-F2 is a value which isdetermined at a time of setting the connection 697, which can be writtenby copying the value of the VCI-F1 of the cell received by the IWU 636,i.e., the VPI_(63M) which is the access address of the CLSF-O 63M.

Next, the IWU 637 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. Here, there is a need to assign theVCI/VPI such that the CLSF-I 63J can identify the cell even when thecells arrive from all the terminals (capable of transmitting datagram tothe CLSF-I 63J) within the network simultaneously.

The datagram arrived at the CLSF-I 63J is cell re-assembled once, andthe ATM connection is terminated. The CLSF-I 63J analyzes the upperlayer address data and recognizes that the destination of the datagramas the terminal 63B. Then, the CLSF-I 63J generates the cell with theVPI-F having VPI_(63B) which is the access address of theterminal_(63B), the VCI-F1 having the VPI_(63J) which is the accessaddress of the CLSF-I 63J, and the VCI-F2 having an identificationnumber assigned to the connection 699, and transmits the datagram to theterminal 63B. Here, the terminal 63B can uniquely identify the datagramto which the cell belongs from the VCI-F1 and the VCI-F2. Namely, thedatagram can be re-assembled at the terminal 63A.

(2) Scheme 2

In this scheme, each CLSF-O carries out the re-assembling of thedatagram, but the address resolution is carried out by each terminal.The address data of the target network obtained by the addressresolution is written in the VCI-F1 of the cell transmitted from theterminal to the CLSF-O. Here, the cell transmission (datagramtransmission) from the CLSF-O cannot be carried out in the pipeline-likemanner, but there is no need for the CLSF-O to carry out the addressresolution of the network to which the destination terminal belongsaccording to the address data in the received datagram.

In this scheme, each IWU, CLSF-O and CLSF-I has obtained the VPIaddress, and all the connections identified by the obtained VPIs are ATMconnections to be used for the cell transmission related to the datagramtransmission (connection-less communication). Namely, each of the CLSFserver and the IWU requires the ATM connection (not connection-less one)in order for itself to carry out the communication with the otherterminal/server. In other words, there is a need for the CLSF to obtainat least two access addresses (VPIs) at a time of the boot.

At this point, the following coding scheme for the VCI field of the cellrelated to the connection-less communication is used. Here, 16 bits ofthe VCI field are defined to form two 8 bits sub-fields VCI-F1 andVCI-F2, but the positions of these sub-fields is not specified.

First, the cell to be transmitted within the external sub-network iscoded as follows. Namely, this coding scheme is the coding of the VCIfield for a case in which the cell is transmitted from the sub-networkof the transmission source terminal to the external sub-network via theIWU. Thus, the identification number of the sub-network of thetransmission source terminal is written in the VCI-F1, while the theidentification number of the transmission source terminal within thesub-network is written in the VCI-F2. For example, as the identificationnumber of the sub-network, it is possible to set the access address(VPI) of each IWU in the network 631 as the identification address ofthe sub-network, in which case the network 631 itself is codedappropriately. Also, as the identification number of the terminal, it ispossible to set the access address (VPI) of the terminal in eachsub-network. For example, the VCI field of the cell for theconnection-less communication which is outputted from the terminal 63B,VCI-F1 can be VPI₆₃₇₋₁ and VCI-F2 can be VPI_(63B).

As for the cell to be transmitted from the CLSF-O to the IWU, theidentification number of the sub-network to which the transmissionsource terminal belongs is written in the VCI-F1, and the identificationnumber of the CLSF-O within the sub-network is written in the VCI-F2.For example, for the cell for the connection-less communication which isoutputted from the CLSF-O 63M, the VCI-F2 field can be VPI_(63M).

As for the cell to be transmitted from the terminal to the CLSF-O, theidentification number of the sub-network to which the destinationterminal belongs is written in the VCI-F1, and the identification numberof the transmission source terminal within the sub-network is written inthe VCI-F2. For example, for the cell for the connection-lesscommunication which is outputted from the CLSF-O 63M, the VCI-F2 fieldcan be VPI_(63B).

By the scheme as in the above, in the procedure for the addressresolution which precedes the datagram transmission, the terminal cantransmits the datagram (one or more cells) as long as the identificationnumber of the sub-network to which the destination terminal belongs.This datagram transmission will now be described in detail.

First, the datagram transmission from the terminal 63A to the terminal63C will be described.

In this case, the terminal 63A transmits to the CLSF-O 63M the cell withthe VPI-F having VPI_(63M) which is the access address of the CLSF-O63M, the VCI-F1 having the identification number of the network 631(which can be the VPI_(63L) which is the access address of the CLSF-I63L), and the VCI-F2 having VPI_(63A) which is the access address of theterminal 63A.

Then, the CLSF-O 63M transmits to the IWU 636 the cell with the VPI-Fhaving VPI₆₃₆₋₂ which is the access address of the IWU 636 for thenetwork 632, the VCI-F1 having the identification number of the network631 (which can be the VPI_(63L) which is the access address of theCLSF-1 63L) that can be copied directly from the VCI-F1 of the receivedcell, and the VCI-F2 having VPI_(63M) which is the access address of theCLSF-O 63M.

The IWU 636 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI_(63L) which is the access address of the CLSF-I 63L, theVPI-F of the cell to be outputted can be obtained by copying the VCI-F1of the received cell which is VPI_(63L). Then, to the VCI-F1, VPI₆₃₆₋₁which is the access address of the IWU 636 for the network 631 iswritten. Here, the VCI-F2 can be set to be transparent. Namely, in acase the VCI-F1 of the cell to be transmitted from the CLSF-O 63M hasVPI_(63L), the relaying of the cell can be realized by the procedure of(1) copying the VCI-F1 of the received cell to the VPI-F of thetransmission cell, (2) copying the VCI-F2 of the received cell to theVCI-F2 of the transmission cell, and (3) writing VPI₆₃₆₋₁ to the VCI-F1of the transmission cell.

The datagram arrived at the CLSF-I 63L is cell re-assembled once, andthe ATM connection is terminated. The CLSF-I 63L analyzes the upperlayer address data and recognizes that the destination of the datagramas the terminal 63C. Then, the CLSF-I 63L generates the cell with theVPI-F having VPI_(63C) which is the access address of the terminal 63C,and transmits the datagram to the terminal 63C. Here, the terminal 63Ccan uniquely identify the datagram to which the cell belongs from theVCI-F1 and the VCI-F2. Namely, the datagram can be re-assembled at theterminal 63C.

Next, the datagram transmission from the terminal 63A to the publicnetwork 635 will be described.

In this case, the terminal 63A transmits to the CLSF-O 63M the cell withthe VPI-F having VPI_(63M) which is the access address of the CLSF-O63M, the VCI-F1 having the identification number of the public network635 (which can be the VPI₆₃₉ which is the access address of the IWU639), and the VCI-F2 having VPI_(63A) which is the access address of theterminal 63A.

Then, the CLSF-O 63M transmits to the IWU 636 the cell with the VPI-Fhaving VPI₆₃₆₋₂ which is the access address of the IWU 636 for thenetwork 632, the VCI-F1 having the identification number of the publicnetwork 635 (which can be the VPI₆₃₉ which is the access address of theIWU 639) that can be copied directly from the VCI-F1 of the receivedcell, and the VCI-F2 having VPI_(63M) which is the access address of theCLSF-O 63M.

The IWU 636 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI₆₃₉ which is the access address of the IWU 639, the VPI-F ofthe cell to be outputted can be obtained by copying the VCI-F1 of thereceived cell. Then, to the VCI-F1, VPI₆₃₆₋₁ which is the access addressof the IWU 636 for the network 631 is written. Here, the VCI-F2 can beset to be transparent. Namely, in a case the VCI-F1 of the cell to betransmitted from the CLSF-O 63M has VPI₆₃₉, the relaying of the cell canbe realized by the procedure of (1) copying the VCI-F1 of the receivedcell to the VPI-F of the transmission cell, (2) copying the VCI-F2 ofthe received cell to the VCI-F2 of the transmission cell, and (3)writing VPI₆₃₆₋₁ to the VCI-F1 of the transmission cell.

The IWU 639 writes the VCI/VPI assigned to the ATM connection 696defined by the public network 635 according to the VCI data of thereceived cell, and transmits the cell to the public network 635.

Finally, the datagram transmission from the terminal 63A to the terminal63B will be described.

In this case, the terminal 63A transmits to the CLSF-O 63M the cell withthe VPI-F having VPI_(63M) which is the access address of the CLSF-O63M, the VCI-F1 having the identification number of the network 633(which can be the VPI_(63J) which is the access address of the CLSF-I63J or the VPI₆₃₇ which is the access address of the IWU 637, and theVCI-F2 having VPI_(63A) which is the access address of the terminal 63A.

Then, the CLSF-O 63M transmits to the IWU 636 the cell with the VPI-Fhaving VPI₆₃₆₋₂ which is the access address of the IWU 636 for thenetwork 632, the VCI-F1 having the identification number of the network633 (which can be the VPI₆₃₇₋₁ which is the access address of the IWU637 for the network 631) that can be directly copied from the VCI-F1 ofthe received cell, and the VCI-F2 having VPI_(63M) which is the accessaddress of the CLSF-O 63M.

The IWU 636 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI₆₃₇₋₁ which is the access address of the IWU 637 for thenetwork 631, the VPI-F of the cell to be outputted can be obtained bycopying the VCI-F1 of the received cell. Then, to the VCI-F1, VPI₆₃₆₋₁which is the access address of the IWU 636 for the network 631 iswritten. Here, the VCI-F2 can be set to be transparent. Namely, in acase the VCI-F1 of the cell to be transmitted from the CLSF-O 63M hasVPI₆₃₇₋₁, the relaying of the cell can be realized by the procedure of(1) copying the VCI-F1 of the received cell to the VPI-F of thetransmission cell, (2) copying the VCI-F2 of the received cell to theVCI-F2 of the transmission cell, and (3) writing VPI₆₃₆₋₁ to the VCI-F1of the transmission cell.

Then, the IWU 637 analyzes the VCI of the received cell, as well as thecorresponding VPI-F, VCI-F1, and the VCI-F2. The VCI field of the cellto be transmitted from the IWU 637 to the CLSF-I 63J can set the VCIfield data of the received cell to be transparent. Namely, it ispossible to make the VCI-F1 having VPI₆₃₆₋₁ and the VCI-F2 havingVPI_(63J).

The datagram arrived at the CLSF-I 63J is cell re-assembled once, andthe ATM connection is terminated. The CLSF-I 63J analyzes the upperlayer address data and recognizes that the destination of the datagramas the terminal 63B. Then, the CLSF-I 63J generates the cell with theVPI-F having VPI_(63B) which is the access address of the terminal 63D,and transmits the datagram to the terminal 63D. Here, the terminal 63Bcan uniquely identify the datagram to which the cell belongs from theVCI-F1 and the VCI-F2. Namely, the datagram can be re-assembled at theterminal 63B.

(3) Scheme 3

This scheme is a modification of the scheme 2 described above in whichthe pipeline type transmission of the cell belonging to the datagram isrealized.

In this scheme, each CLSF-O carries out the re-assembling of thedatagram, but the address resolution is carried out by each terminal, sothat there is no need for the CLSF-O to carry out the address resolutionof the network to which the destination terminal belongs according tothe address data within the received datagram. The address data of thetarget network obtained by the address resolution is written in theVCI-F1 of the cell transmitted from the terminal to the CLSF-O.

Here, the following coding scheme for the VCI field of the cell relatedto the connection-less communication is used.

First, the cell to be transmitted within the external sub-network iscoded in the similar manner as in the Scheme 2 described above. Namely,the VPI-F1 has the identification number of the sub-network of thetransmission source terminal, while the VCI-F2 has the identificationnumber of the transmission source terminal within the sub-network.

As for the cell to be transmitted from the CLSF-O to the IWU, theidentification number of the sub-network to which the destinationterminal belongs is written in the VCI-F1, and the identification numberof the transmission source terminal within the sub-network is written inthe VCI-F2. For example, for the cell concerning the datagram outputtedfrom the terminal 63A, the VCI-F2 field can be VPI_(63A).

As for the cell to be transmitted from the terminal to the CLSF-O, it iscoded in the similar manner as in the Scheme 2 described above. Namely,the identification number of the sub-network to which the destinationterminal belongs is written in the VCI-F1, and the identification numberof the transmission source terminal within the sub-network is written inthe VCI-F2.

Using this coding scheme, the datagram transmission can be carried outas follows. First, the datagram transmission from the terminal 63A tothe terminal 63C will be described.

In this case, the terminal 63A transmits to the CLSF-O 63M the cell withthe VPI-F having VPI_(63M) which is the access address of the CLSF-O63M, the VCI-F1 having the identification number of the network 631(which can be the VPI_(63L) which is the access address of the CLSF-I63L), and the VCI-F2 having VPI_(63M) which is the access address of theCLSF-O 63M.

Then, the CLSF-O 63M transmits to the IWU 636 the cell with the VPI-Fhaving VPI₆₃₆₋₂ which is the access address of the IWU 636 for thenetwork 632, along with the VCI-F1 and the VCI-F2 copied directly fromthe data in the received cell. Namely, the VCI-F1 has the identificationnumber of the network 631 (which can be the VPI_(63L) which is theaccess address of the CLSF-I 63L), and the VCI-F2 has VPI_(63A) which isthe access address of the terminal 63A. Here, it is possible to carryout the pipeline type transmission of the cells to the IWU 636sequentially, without carrying out the re-assembling of the datagram atthe CLSF-O 63M.

The IWU 636 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI_(63L) which is the access address of the CLSF-I 63L, theVPI-F of the cell to be outputted can be obtained by copying the VCI-F1of the received cell which is VPI_(63L). Then, to the VCI-F1, VPI₆₃₆₋₁which is the access address of the IWU 636 for the network 631 iswritten. Here, the VCI-F2 can be set to be transparent. Namely, in acase the VCI-F1 of the cell to be transmitted from the CLSF-O 63M hasVPI_(63L), the relaying of the cell can be realized by the procedure of(1) copying the VCI-F1 of the received cell to the VPI-F of thetransmission cell, (2) copying the VCI-F2 of the received cell to theVCI-F2 of the transmission cell, and (3) writing VPI₆₃₆₋₁ to the VCI-F1of the transmission cell.

The datagram arrived at the CLSF-I 63L is cell re-assembled once, andthe ATM connection is terminated. The CLSF-I 63L analyzes the upperlayer address data and recognizes that the destination of the datagramas the terminal 63C. Then, the CLSF-I 63L generates the cell with theVPI-F having VPI_(63C) which is the access address of the terminal 63C,and transmits the datagram to the terminal 63C. Here, the terminal 63Ccan uniquely identify the datagram to which the cell belongs from theVCI-F1 and the VCI-F2. Namely, the datagram can be re-assembled at theterminal 63C.

Next, the datagram transmission from the terminal 63A to the publicnetwork 635 will be described.

In this case, the terminal 63A transmits to the CLSF-O 63M the cell withthe VPI-F having VPI_(63M) which is the access address of the CLSF-O63M, the VCI-F1 having the identification number of the public network635 (which can be the VPI₆₃₉ which is the access address of the IWU639), and the VCI-F2 having VPI_(63A) which is the access address of theterminal 63A.

Then, the CLSF-O 63M transmits to the IWU 636 the cell with the VPI-Fhaving VPI₆₃₆₋₂ which is the access address of the IWU 636 for thenetwork 632, along with the VCI-F1 and the VCI-F2 copied directly fromthe data in the received cell. Namely, the VCI-F1 has the identificationnumber of the public network 635 (which can be the VPI₆₃₉ which is theaccess address of the IWU 639), and the VCI-F2 has VPI_(63A) which isthe access address of the terminal 63A. Here, it is possible to carryout the pipeline type transmission of the cells to the IWU 636sequentially, without carrying out the re-assembling of the datagram atthe CLSF-O 63M.

The IWU 636 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI₆₃₉ which is the access address of the IWU 639, the VPI-F ofthe cell to be outputted can be obtained by copying the VCI-F1 of thereceived cell. Then, to the VCI-F1, VPI₆₃₆₋₁ which is the access addressof the IWU 636 for the network 631 is written. Here, the VCI-F2 can beset to be transparent. Namely, in a case the VCI-F1 of the cell to betransmitted from the CLSF-O 63M has VPI₆₃₉, the relaying of the cell canbe realized by the procedure of (1) copying the VCI-F1 of the receivedcell to the VPI-F of the transmission cell, (2) copying the VCI-F2 ofthe received cell to the VCI-F2 of the transmission cell, and (3)writing VPI₆₃₆₋₁ to the VCI-F1 of the transmission cell.

The IWU 639 writes the VCI/VPI assigned to the ATM connection 696defined by the public network 635 according to the VCI data of thereceived cell, and transmits the cell to the public network 635.

Finally, the datagram transmission from the terminal 63A to the terminal63B will be described.

In this case, the terminal 63A transmits to the CLSF-O 63M the cell withthe VPI-F having VPI_(63M) which is the access address of the CLSF-O63M, the VCI-F1 having the identification number of the network 633(which can be the VPI_(63J) which is the access address of the CLSF-I63J), and the VCI-F2 having VPI_(63A) which is the access address of theterminal 63A.

Then, the CLSF-O 63M transmits to the IWU 636 the cell with the VPI-Fhaving VPI₆₃₆₋₂ which is the access address of the IWU 636 for thenetwork 632, along with the VCI-F1 and VCI-F2 copied directly from thedata in the received cell. Thus, the VCI-F1 has the identificationnumber of the network 633 (which can be the VPI₆₃₇₋₁ which is the accessaddress of the IWU 637 for the network 631), and the VCI-F2 hasVPI_(63A) which is the access address of the terminal 63A. Here, it ispossible to carry out the pipeline type transmission of the cells to theIWU 636 sequentially, without carrying out the re-assembling of thedatagram at the CLSF-O 63M.

The IWU 636 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI₆₃₇₋₁ which is the access address of the IWU 637 for thenetwork 631, the VPI-F of the cell to be outputted can be obtained bycopying the VCI-F1 of the received cell. Then, to the VCI-F1, VPI₆₃₆₋₁which is the access address of the IWU 636 for the network 631 iswritten. Here, the VCI-F2 can be set to be transparent. Namely, in acase the VCI-F1 of the cell to be transmitted from the CLSF-O 63M hasVPI₆₃₇₋₁, the relaying of the cell can be realized by the procedure of(1) copying the VCI-F1 of the received cell to the VPI-F of thetransmission cell, (2) copying the VCI-F2 of the received cell to theVCI-F2 of the transmission cell, and (3) writing VPI₆₃₆₋₁ to the VCI-F1of the transmission cell.

Then, the IWU 637 analyzes the VCI of the received cell, as well as thecorresponding VPI-F, VCI-F1, and the VCI-F2. The VCI field of the cellto be transmitted from the IWU 637 to the CLSF-I 63J can set the VCIfield data of the received cell to be transparent. Namely, it ispossible to make the VCI-F1 having VPI₆₃₆₋₁ and the VCI-F2 havingVPI_(63J).

The datagram arrived at the CLSF-I 63J is cell re-assembled once, andthe ATM connection is terminated. The CLSF-I 63J analyzes the upperlayer address data and recognizes that the destination of the datagramas the terminal 63B. Then, the CLSF-I 63J generates the cell with theVPI-F having VPI_(63B) which is the access address of the terminal 63D,and transmits the datagram to the terminal 63D. Here, the terminal 63Bcan uniquely identify the datagram to which the cell belongs from theVCI-F1 and the VCI-F2. Namely, the datagram can be re-assembled at theterminal 63B.

(4) Scheme 4

This scheme is another modification of the scheme 2 described above inwhich the pipeline type transmission of the datagram to the differentsub-network is realized. In this scheme, the datagram transmission fromone terminal belonging to one sub-network to another terminal belongingto the same sub-network cannot be carried out in the pipeline-likemanner, but the pipeline type transmission is possible for thedestination terminal belonging to the different sub-network. In thiscase, it is necessary for the CLSF-O to have some buffer space.

In this scheme, each CLSF-O carries out the re-assembling of thedatagram, but the address resolution is carried out by each terminal, sothat there is no need for the CLSF-O to carry out the address resolutionof the network to which the destination terminal belongs according tothe address data within the received datagram. The address data of thetarget network obtained by the address resolution is written in theVCI-F1 of the cell transmitted from the terminal to the CLSF-O.

Here, the datagram transmission at the CLSF-O is carried out by thefollowing procedure.

Step 1: The cell (a first cell for the datagram) is received from theterminal.

Step 2: The address data of the target sub-network written in the VCI-F1is analyzed.

Step 3: Whether there is currently carried out datagram transmissionwith respect to the analyzed target sub-network or not is checked.

Step 4: The end of the datagram transmission can be recognized by theCLSF-O according to the identification codes above the ATM layer such asthose of the payload type coding in the AAL 5 for example. In a casethere is no other datagram transmission with respect to the analyzedtarget sub-network, the received cell is relayed to the targetsub-network. Here, the received cell is transmitted in the pipeline-likemanner, i.e., without being re-assembled once at the CLSF-O.

Step 5: In a case there is the other datagram transmission with respectto the analyzed target sub-network from the other terminal, the receivedcell is stored in the buffer until that other datagram transmission iscompleted. When the completion of that other datagram transmission isconfirmed, the cells stored in the buffer are transmitted sequentially.At this point, there is no need for the cells (datagrams) stored in thebuffer to be re-assembled, and the transmission of the first cell can bestarted before the last cell of the cells belonging to the datagramsstored in the buffer arrives. Also, by defining the appropriate protocolbetween the CLSF-O and the terminal, it is also possible to carry outthe flow control so as not to cause the discarding of the stored cellsdue to the overflow.

Here, the following coding scheme for the VCI field of the cell relatedto the connection-less communication is used.

First, the cell to be transmitted within the external sub-network iscoded in the similar manner as in the Scheme 2 described above. Namely,the VPI-F1 has the identification number of the sub-network of thetransmission source terminal, while the VCI-F2 has the identificationnumber of the transmission source terminal within the sub-network.

As for the cell to be transmitted from the CLSF-O to the IWU, theidentification number of the sub-network to which the destinationterminal belongs is written in the VCI-F1, and the identification numberof the transmission source network is written in the VCI-F2. Forexample, for the cell concerning the datagram outputted from theterminal 63A, the VCI-F2 field can be VPI₆₃₆₋₁.

As for the cell to be transmitted from the terminal to the CLSF-O, it iscoded in the similar manner as in the Scheme 2 described above. Namely,the identification number of the sub-network to which the destinationterminal belongs is written in the VCI-F1, and the identification numberof the transmission source terminal within the sub-network is written inthe VCI-F2.

Using this coding scheme, the datagram transmission can be carried outas follows. First, the datagram transmission from the terminal 63A tothe terminal 63C will be described.

In this case, the terminal 63A transmits to the CLSF-O 63M the cell withthe VPI-F having VPI_(63M) which is the access address of the CLSF-O63M, the VCI-F1 having the identification number of the network 631(which can be the VPI_(63L) which is the access address of the CLSF-I63L), and the VCI-F2 having VPI_(63M) which is the access address of theCLSF-O 63M.

Then, when there is no other datagram transmission to the sub-network631, the CLSF-O 63M transmits to the IWU 636 the cell with the VPI-Fhaving VPI₆₃₈₋₂ which is the access address of the IWU 636 for thenetwork 632, the VCI-F1 having the identification number of the network631 (which can be the VPI_(63L) which is the access address of theCLSF-I 63L) that can be directly copied from the data in the receivedcell, and the VCI-F2 having VPI₆₃₆₋₂ which is the access address of theIWU 636 for the terminal 631 which represents the identification numberof the sub-network 632 for example. Here, the VCI-F2 can be anyidentifier for identifying its own sub-network in this case. Also, it ispossible to carry out the pipeline type transmission of the cells to theIWU 636 sequentially, without carrying out the re-assembling of thedatagram at the CLSF-O 63M.

The IWU 636 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI_(63L) which is the access address of the CLSF-I 63L, theVPI-F of the cell to be outputted can be obtained by copying the VCI-F1of the received cell which is VPI_(63L). Then, to the VCI-F1, VPI₆₃₆₋₁which is the access address of the IWU 636 for the network 631 iswritten. Here, the VCI-F2 can be set to be transparent. Namely, in acase the VCI-F1 of the cell to be transmitted from the CLSF-O 63M hasVPI_(63L), the relaying of the cell can be realized by the procedure of(1) copying the VCI-F1 of the received cell to the VPI-F of thetransmission cell, (2) copying the VCI-F2 of the received cell to theVCI-F2 of the transmission cell, and (3) writing VPI₆₃₆₋₁ to the VCI-F1of the transmission cell.

The datagram arrived at the CLSF-I 63L is cell re-assembled once, andthe ATM connection is terminated. The CLSF-I 63L analyzes the upperlayer address data and recognizes that the destination of the datagramas the terminal 63C. Then, the CLSF-I 63L generates the cell with theVPI-F having VPI_(63C) which is the access address of the terminal 63C,and transmits the datagram to the terminal 63C. Here, the terminal 63Ccan uniquely identify the datagram to which the cell belongs from theVCI-F1 and the VCI-F2. Namely, the datagram can be re-assembled at theterminal 63C.

Next, the datagram transmission from the terminal 63A to the publicnetwork 635 will be described.

In this case, the terminal 63A transmits to the CLSF-O 63M the cell withthe VPI-F having VPI_(63M) which is the access address of the CLSF-O63M, the VCI-F1 having the identification number of the public network635 (which can be the VPI₆₃₉ which is the access address of the IWU639), and the VCI-F2 having VPI_(63A) which is the access address of theterminal 63A.

Then, when there is not other datagram transmission to the publicnetwork 635, the CLSF-O 63M transmits to the IWU 636 the cell with theVPI-F having VPI₆₃₆₋₂ which is the access address of the IWU 636 for thenetwork 632, the VCI-F1 having the identification number of the publicnetwork 635 (which can be the VPI₆₃₉ which is the access address of theIWU 639) that can be directly copied from the data in the received cell,and the VCI-F2 having VPI₆₃₆₋₂ which is the access address of the IWU636 for the terminal 631 which represents the identification number ofthe sub-network 632. Here, the VCI-F2 can be any identifier foridentifying its own sub-network in this case. Also, it is possible tocarry out the pipeline type transmission of the cells to the IWU 636sequentially, without carrying out the re-assembling of the datagram atthe CLSF-O 63M.

The IWU 636 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI₆₃₉ which is the access address of the IWU 639, the VPI-F ofthe cell to be outputted can be obtained by copying the VCI-F1 of thereceived cell. Then, to the VCI-F1, VPI₆₃₆₋₁ which is the access addressof the IWU 636 for the network 631 is written. Here, the VCI-F2 can beset to be transparent. Namely, in a case the VCI-F1 of the cell to betransmitted from the CLSF-O 63M has VPI₆₃₉, the relaying of the cell canbe realized by the procedure of (1) copying the VCI-F1 of the receivedcell to the VPI-F of the transmission cell, (2) copying the VCI-F2 ofthe received cell to the VCI-F2 of the transmission cell, and (3)writing VPI₆₃₆₋₁ to the VCI-F1 of the transmission cell.

The IWU 639 writes the VCI/VPI assigned to the ATM connection 696defined by the public network 635 according to the VCI data of thereceived cell, and transmits the cell to the public network 635.

Finally, the datagram transmission from the terminal 63A to the terminal63B will be described.

In this case, the terminal 63A transmits to the CLSF-O 63M the cell withthe VPI-F having VPI_(63M) which is the access address of the CLSF-O63M, the VCI-F1 having the identification number of the network 633(which can be the VPI₆₃₇₋₁ which is the access address of the IWU 637for the network 631), and the VCI-F2 having VPI_(63A) which is theaccess address of the terminal 63A.

Then, when there is no other datagram transmission to the sub-network633, the CLSF-O 63M transmits to the IWU 636 the cell with the VPI-Fhaving VPI₆₃₆₋₂ which is the access address of the IWU 636 for thenetwork 632, the VCI-F1 having the identification number of the network633 (which can be the VPI₆₃₇₋₁ which is the access address of the IWU637 for the network 631) that can be directly copied from the data inthe received cell, and the VCI-F2 having VPI₆₃₈₋₂ which is the accessaddress of the IWU 636 for the terminal 631 which represents theidentification number of the sub-network 632. Here, the VCI-F2 can beany identifier for identifying its own sub-network in this case. Also,it is possible to carry out the pipeline type transmission of the cellsto the IWU 636 sequentially, without carrying out the re-assembling ofthe datagram at the CLSF-O 63M.

The IWU 636 analyzes the VCI of the received cell, as well as thecorresponding VPI-F and the VCI-F2. For example, in a case the VCI-F1has the VPI₆₃₇₋₁ which is the access address of the IWU 637 for thenetwork 631, the VPI-F of the cell to be outputted can be obtained bycopying the VCI-F1 of the received cell. Then, to the VCI-F1, VPI₆₃₈₋₁which is the access address of the IWU 636 for the network 631 iswritten. Here, the VCI-F2 can be set to be transparent. Namely, in acase the VCI-F1 of the cell to be transmitted from the CLSF-O 63M hasVPI₆₃₇₋₁, the relaying of the cell can be realized by the procedure of(1) copying the VCI-F1 of the received cell to the VPI-F of thetransmission cell, (2) copying the VCI-F2 of the received cell to theVCI-F2 of the transmission cell, and (3) writing VPI₆₃₆₋₁ to the VCI-F1of the transmission cell.

Then, the IWU 637 analyzes the VCI of the received cell, as well as thecorresponding VPI-F, VCI-F1, and the VCI-F2. The VCI field of the cellto be transmitted from the IWU 637 to the CLSF-I 63J can set the VCIfield data of the received cell to be transparent. Namely, it ispossible to make the VCI-F1 having VPI₆₃₆₋₁ and the VCI-F2 havingVPI_(63J).

The datagram arrived at the CLSF-I 63J is cell re-assembled once, andthe ATM connection is terminated. The CLSF-I 63J analyzes the upperlayer address data and recognizes that the destination of the datagramas the terminal 63B. Then, the CLSF-I 63J generates the cell with theVPI-F having VPI_(63B) which is the access address of the terminal 63D,and transmits the datagram to the terminal 63D. Here, the terminal 63Bcan uniquely identify the datagram to which the cell belongs from theVCI-F1 and the VCI-F2. Namely, the datagram can be re-assembled at theterminal 63B.

DATAGRAM DELIVERY IN NETWORK WITH FLAT TOPOLOGY

Next: various schemes for the datagram delivery to the destinationterminal in the network with flat topology will be described.

FIG. 34 shows a system configuration of the network with the falttopology, in which a plurality of sub-networks 701 to 706 with theinter-networking provided by the IWUs 70D, 70E, 70F, 70G, 70H, and 70J.In addition, the public networks 707 and 708 are connected with thenetworks 703 and 704 through the IWUs 70K and 70M, respectively. Here,each IWU can realize the relaying of the ATM cells without terminatingthe ATM connection, by having a function to convert the VCI/VPI of thereceived cell into VCI/VPI assigned to the corresponding ATM connectionin the neighboring network.

In this configuration of FIG. 34, the connection lines among thenetworks 701 to 706 may be the connection lines within a campus, or thededicated lines or switched lines of the public networks. Also, thepublic networks 707 and 708 are provided so that the accesses to theterminals and the networks other than those defined within this systemcan be made through these public networks 707 and 708. Thus, acommunication with the terminal connected to the general public networkcan be made via these public networks 707 and 708.

Furthermore, the networks 701 to 706 are provided with the CLSFs 7011,7021, 7031, 7041, 7051, and 7061, respectively, such that the cellhandling related to the connection-less communication can be realized.

Here, in this network with the flat topology, there is no limitationconcerning the distance of the network, i.e., an index indicating howmany relaying points (sub-networks) must be required in joiningarbitrary points within the network at most, in contrast to the abovedescribed network with the hierarchical topology in which the targetsub-network can be reached via at most one sub-network.

Also, in this case, the manner of realizing the address resolution isbasically equivalent to that in the hierarchical network describedabove, so that the description concerning the manner of constructing theATM connections related to the address resolution will not be repeatedhere. Namely, there are two schemes including a scheme in which all theaddress resolution servers are equal and analyze the address data of thenetwork independently, and a scheme in which the address data aremanaged and analyzed by the logical hierarchical structure among theaddress resolution servers. As for the ATM connections to be set upamong the address resolution servers, they can have any desiredstructure ranging from the full meshed structure to the minimallyspanning tree structure.

1. General ATM Network: Scheme I

In this case, the ATM connections are provided as follows. Namely, inorder to transmit the datagram from the terminal connected to eachsub-network to the terminal connected to an optical sub-network, the ATMconnections are set up from each IWU to the CLSFs provided within allthe sub-networks. That is, the single direction ATM connections from theIWU to the CLSFs are set up in the fully meshed manner. Here, the IWU(relaying IWU) provided on the route of the ATM connection carries outthe rewriting of the ATM header data or at least the VCI/VPI conversion,and the cell relaying is executed as the work at the ATM layer. In otherwords, the ATM connection is not terminated in principle at the IWU.Also, it is also possible for the CLSF to be provided at a position ofthe IWU if desired.

Here, when the datagram is transmitted to the network defined by thepublic networks 707 and 708, the received cell to which the datagrambelongs is transmitted to the server for terminating the datagramconnection of the public network once. This server can also be providedon the IWU if desired. The server which terminated the datagramtransmitted from the public network relays the datagram by the sameprocedure as the datagram transmission from the terminal within thedefined network.

The exemplary ATM connection configuration at a time of transmitting thedatagram from the public network 707 to the terminal 70A is shown inFIG. 35. In this exemplary case, the datagram transmitted from thepublic network 707 is terminated at the CLSF-P 811 once, and thentransmitted to the terminal 70A via the CLSF 7011. Therefore, there arethree ATM connections 812, 813, and 814.

In this case, the procedure for transmitting the datagram of theconnection-less communication from one terminal to the destinationterminal is as follows.

(1) The terminal makes the AR request. This can be done either always oronly in a case the address resolution cannot be made by the terminalitself such as a case in which a suitable entry is present in the ARtable.

(2) The terminal obtains the VCI/VPI data which is the identifier of theATM connection provided from the ARS in order to make an access to thedestination terminal.

(3) The terminal attaches the obtained VCI/VPI to the cell and outputsthe cell to the network, so as to complete the datagram transmission.

Here, at a time of the datagram transmission, there is no need for theterminal to carry out the procedure for setting up a particularconnection defined in the ATM network.

Next, the routing to the destination terminal in this case will bedescribed. The final delivery of the datagram to each terminal iscarried out by each CLSF only for its own network. For example, the CLSF7061 carries out the datagram delivery to only the terminals belongingto the network 706. Similarly, the CLSF 7031 carries out the datagramdelivery to only the terminals within the network 703. When the networkaddress possessed by the datagram received at each CLSF is not presentin the address entries possessed by that CLSF, or when the address ofthe received datagram is not elements of the network address space ofthe network, it is judged that the datagram has been transmittedincorrectly. The treatment of the erroneously delivered datagram willnot be discussed here.

Thus, it suffices for each CLSF to possess only the address data of theterminals of the network to which that CLSF itself belongs. When theaddress of the of the received datagram is present in its own network,the appropriate ATM connection is selected and the relaying of thedatagram is carried out.

An exemplary protocol processing in a case of transmitting the datagramfrom the public network 707 to the terminal 70A through the ATMconnections of FIG. 35 described above is shown in FIG. 36. Similarly,an exemplary protocol processing in a case of transmitting the datagramfrom the terminal 70A to the terminal 70B through the ATM connections ofFIG. 37 is shown in FIG. 38 for the datagram relayed through paths 771to 776 shown in FIG. 39. In this case, the ATM connection is transmittedat the CLSF 7061 once, i.e., the protocol of the OSI layer 3 isterminated at the CLSF 7061. The OSI layer 3 protocol processing iscarried out at the CLSF 7061, and the data unit is transmitted to theterminal 70B using the ATM connection. In this manner, at a time of thedatagram transmission to the terminal other than those of its ownsub-network, the end-to-end datagram delivery can be realized with onlyone ATM connection termination.

Similarly, an exemplary protocol processing in a case of transmittingthe datagram from the terminal 70A to the public network 708 through theATM connections of FIG. 40 is shown in FIG. 41 for the datagram relayedthrough paths 781 to 783 as shown in FIG. 42.

Now, more concrete example of this case will be described in detail.

In transmitting the datagram from the terminal 70A to the terminal 70B,the required ATM connection configuration is as shown in FIG. 37. Here,at a time of the datagram transmission, the terminal 70A carries out theaddress resolution of the terminal 70B by analyzing the access addressdata of the sub-network to which the terminal 70B belongs. Namely, theterminal 70A transmits the AR request message containing the addressdata of the terminal 70B to the ARS. The ARS which received this ARrequest then carries out the address resolution and returns to theterminal 70A the AR response indicating the VCI/VPI data fortransmitting the cell to the terminal 70B.

The terminal 70A which obtained the ATM layer address (VCI/VPI) data fortransmitting the cell to the terminal 70B then outputs the cell attachedwith the VCI/VPI to the network. Then, after the VCI/VPI conversion atthe IWU 70D, the cell is transmitted to the IWU 70G. Similarly, the cellis transmitted to the CLSF 7061 via the IWUs 70H and 70J. The CLSF 7061which received this cell then analyzes the layer 3 address data of thedatagram and transmits the cell to the terminal 70B. Here, the celltransmission from the CLSF 7061 to the terminal 70B may be made afterall the cells belonging to the datagram are received by the CLSF 7061,or by the pipeline-like cell relaying after the analysis of the layer 3address data of the datagram.

In transmitting the datagram from the terminal 70A to the public network708, the required ATM connection configuration is as shown in FIG. 40.Here, at a time of the datagram transmission, the terminal 70A carriesout the address resolution of the target terminal by analyzing theaccess address data of the sub-network to which the destination terminalbelongs. Namely, the terminal 70A transmits the AR request messagecontaining the address data of the destination terminal to the ARS. TheARS which received this AR request then carries out the addressresolution and returns to the terminal 70A the AR response indicatingthe VCI/VPI data for transmitting the cell to the destination terminal.

The terminal 70A which obtained the ATM layer address (VCI/VPI) data fortransmitting the cell to the terminal 70B then outputs the cell attachedwith the VCI/VPI to the network. Then, after the VCI/VPI conversion atthe IWU 70D, the cell is transmitted to the IWU 70E. Similarly, the cellis transmitted to the public network 708 via the IWU 70M.

In transmitting the datagram from the public network 707 to the terminal70A, the required ATM connection configuration is as shown in FIG. 35.Here, the transmission source terminal is present within the publicnetwork 707, and the cell related to the connection-less communicationfrom the public network 707 is transmitted to the CLSF-P 811 via the IWU70K. The VCI/VPI conversion table provided at the IWU 70K is set suchthat the cell is transmitted to the CLSF-P 811 whenever the cell havingthe VCI/VPI assigned to the cell used by the connection-lesscommunication in the public network 707 has arrived.

The CLSF-P 811 terminates the ATM connection once, and analyzes thelayer 3 address data of the datagram. In a case the address data to beanalyzed is not present in the table within the CLSF-P 811, the CLSF-P811 transmits the AR request to the ARS. The CLSF-P 811 which obtainedthe ATM layer address (VCI/VPI) data for transmitting the cell to theterminal 70A then outputs the cell attached with the VCI/VPI to thenetwork. After the VCI/VPI conversion at the IWU 70G, the cell istransmitted to the IWU 70D, and then to the CLSF 7011. The CLSF 7011which received the cell then analyzes the layer 3 address data of thedatagram, and transmits the cell to the terminal 70A. Here, the celltransmission from the CLSF 7011 to the terminal 70A and the celltransmission from the CLSF-P 811 to the IWU 70G may be made after allthe cells belonging to the datagram are received by the CLSF 7061 or theCLSF-P 811, or by the pipeline-like cell relaying after the analysis ofthe layer 3 address data of the datagram.

2. General ATM Network: Scheme II

In this case, the ATM connections are provided as follows. Namely, inorder to transmit the datagram from the terminal connected to eachsub-network to the terminal connected to an optical sub-network, the ATMconnections are set up from each CLSF to the CLSFs provided within allthe sub-networks. That is, the ATM connections among the CLSFs are setup in the fully meshed manner. Here, the IWU (relaying IWU) provided onthe route of the ATM connection carries out the rewriting of the ATMheader data or at least the VCI/VPI conversion, and the cell relaying isexecuted as the work at the ATM layer. In other words, the ATMconnection is not terminated in principle at the IWU. Also, it is alsopossible for the CLSF to be provided at a position of the IWU ifdesired.

Here, when the datagram is transmitted to the network defined by thepublic networks 707 and 708, the received cell to which the datagrambelongs is transmitted to the server for terminating the datagramconnection of the public network once. This server can also be providedon the IWU if desired. The server which terminated the datagramtransmitted from the public network relays the datagram by the sameprocedure as the datagram transmission from the terminal within thedefined network.

The exemplary ATM connection configuration at a time of transmitting thedatagram from the public network 707 to the terminal 70A is shown inFIG. 43. In this exemplary case, the datagram transmitted from thepublic network 707 is terminated at the CLSF-P 811 once, and thentransmitted to the terminal 70A via the CLSFs 7031 and 7011. Therefore,there are four ATM connections 831, 832, 833, and 834.

Now, in this case, the protocol at the terminal is as follows. Namely,when the terminal judges that the datagram is destined to the externalsub-network, the terminal transmits the datagram to the CLSF. This CLSFis usually present in the same sub-network as the terminal, but the CLSFmay be provided at the other sub-network such as that of the neighboringnode if desired. Here, the ATM connection is assumed to be already setup between the terminal and the CLSF. Each terminal has the addressspace data (such as address masks) for the sub-network to which itbelongs, so that it is possible for each terminal to judge whether thedestination terminal is the terminal within its own network or theterminal of the external sub-network.

An exemplary protocol processing in a case of transmitting the datagramfrom the public network 707 to the terminal 70A through the ATMconnections of FIG. 43 described above is shown in FIG. 44.

In the network architecture shown in FIG. 34, the datagram delivery canbe realized by any of the following two schemes.

(1) In this scheme, the star-shaped ATM connections are set up frombetween the CLSF and the terminals. The terminal transmits the cell tothe CLSF whenever the datagram transmission is to be carried out. Thedatagram delivery is entirely carried out by the CLSF. Namely, even thecommunication between the terminals within the same sub-network isrealized via the CLSF.

(2) The communication between the terminals within the same sub-networkis realized without the CLSF, while the communication with the terminalof the external network is realized via the CLSF.

Next, the routing to the destination terminal in this case will bedescribed. The final delivery of the datagram to each terminal iscarried out by each CLSF only for its own network. For example, the CLSF7061 carries out the datagram delivery to only the terminals belongingto the network 706. Similarly, the CLSF 7031 carries out the datagramdelivery to only the terminals within the network 703. When the networkaddress possessed by the datagram received at each CLSF is not presentin the address entries possessed by that CLSF, or when the address ofthe received datagram is not elements of the network address space ofthe network, it is judged that that datagram has been transmittedincorrectly. The treatment of the erroneously delivered datagram willnot be discussed here.

Thus, it suffices for each CLSF to possess only the address data of theterminals of the network to which that CLSF itself belongs. When theaddress of the of the received datagram is present in its own network,the appropriate ATM connection is selected and the relaying of thedatagram is carried out.

An exemplary ATM connection configuration in a case of transmitting thedatagram from the terminal 70A to the terminal 70B is shown in FIG. 45for the datagram relayed through the paths 791 to 797 shown in FIG. 46.Namely, the ATM connection is terminated at the CLSFs 7061 and 7011. TheOSI layer 3 protocol processing is carried out at the CLSF 7011, and thedata unit is transmitted to the IWU 70D using the ATM connection. Also,the OSI layer 3 protocol processing is carried out at the CLSF 7061, andthe data unit is transmitted to the terminal 70B using the ATMconnection. In this manner, at a time of the datagram transmission tothe terminal other than those of its own sub-network, the end-to-enddatagram delivery can be realized with only two ATM connectionterminations.

Similarly, an exemplary ATM connection configuration in a case oftransmitting the datagram from the terminal 70A to the public network708 is shown in FIG. 47 for the datagram relayed through the paths 801to 804 shown in FIG. 48.

Now, more concrete example of this case will be described in detail.

In transmitting the datagram from the terminal 70A to the terminal 70B,the required ATM connection configuration is as shown in FIG. 45. Here,at a time of the datagram transmission, the terminal 70A transmits thedatagram to the CLSF 7011 when it is analyzed that the terminal 70B isthe terminal belonging to the external sub-network. The CLSF 7011 thenanalyzes the address data of the received datagram, and transmits the ARrequest cell having the address data of the terminal 70B to the ARS whenit does not possess the ATM layer address data for transmitting thedatagram to the terminal 70B (CLSF 7061). The ARS which received this ARrequest then carries out the address resolution and returns to the CLSF7011 the AR response indicating the VCI/VPI data for transmitting thecell to the CLSF 7061 (terminal 70B).

The CLSF 7011 which obtained the ATM layer address (VCI/VPI) data fortransmitting the cell to the terminal 70B then outputs the cell attachedwith the VCI/VPI to the network. Then, after the VCI/VPI conversion atthe IWU 70D, the cell is transmitted to the IWU 70G. Similarly, the cellis transmitted to the CLSF 7061 via the IWUs 70H and 70J. The CLSF 7061which received this cell then analyzes the layer 3 address data of thedatagram and transmits the cell to the terminal 70B. Here, the celltransmission from the CLSF 7061 to the terminal 70B may be made afterall the cells belonging to the datagram are received by the CLSF 7061,or by the pipeline-like cell relaying after the analysis of the layer 3address data of the datagram.

In transmitting the datagram from the terminal 70A to the public network708, the required ATM connection configuration is as shown in FIG. 47.Here, at a time of the datagram transmission, the terminal 70A carriesout the address resolution of the target terminal by analyzing theaccess address data of the sub-network to which the destination terminalbelongs. Namely, the terminal 70A transmits the AR request messagecontaining the address data of the destination terminal to the ARS whenthe terminal 70A cannot carry out the resolution of the address data ofthe destination terminal. The ARS which received this AR request thencarries out the address resolution and returns to the terminal 70A theaccess address data of the CLSF 7011 with the AR response indicating theVCI/VPI data for transmitting the cell to the destination terminal.Then, when it is analyzed that the destination terminal is the terminalbelonging to the external sub-network, the terminal 70A attaches theVCI/VPI data either received or obtained from the analysis, andtransmits the cell to the CLSF 7011.

The CLSF 7011 then analyzes the address data of the received datagram,and transmits the AR request cell having the address data of thedestination terminal to the ARS when it does not possess the ATM layeraddress data for transmitting the datagram to the destination terminal.The ARS which received this AR request then carries out the addressresolution and returns to the CLSF 7011 the AR response indicating theVCI/VPI data for transmitting the cell to the IWU 70M.

The CLSF 7011 which obtained the ATM layer address (VCI/VPI) data fortransmitting the cell to the terminal 70B then outputs the cell attachedwith the VCI/VPI to the network. Then, after the VCI/VPI conversion atthe IWU 70D, the cell is transmitted to the IWU 70E. Similarly, the cellis transmitted to the public network 708 via the IWU 70M.

In transmitting the datagram from the public network 707 to the terminal70A, the required ATM connection configuration is as shown in FIG. 43.Here, the transmission source terminal is present within the publicnetwork 707, and the cell related to the connection-less communicationfrom the public network 707 is transmitted to the CLSF-P 811 via the IWU70K. The VCI/VPI conversion table provided at the IWU 70K is set suchthat the cell is transmitted to the CLSF-P 811 whenever the cell havingthe VCI/VPI assigned to the cell used by the connection-lesscommunication in the public network 707 has arrived.

The CLSF-P 811 terminates the ATM connection once, and analyzes thelayer 3 address data of the datagram. In a case the address data to beanalyzed is not present in the table within the CLSF-P 811, the CLSF-P811 transmits the AR request to the ARS. The CLSF-P 811 which obtainedthe ATM layer address (VCI/VPI) data for transmitting the cell to theCLSF 7031 then outputs the cell attached with the VCI/VPI to thenetwork. The CLSF 7013 then analyzes the address data of the receivedatagram

The CLSF 7031 then analyzes the address data of the received datagram,and transmits the AR request cell having the address data of theterminal 70A to the ARS when it does not possess the ATM layer addressdata for transmitting the datagram to the terminal 70A (CLSF 7011). TheARS which received this AR request then carries out the addressresolution and returns to the CLSF 7031 the AR response indicating theVCI/VPI data for transmitting the cell to the CLSF 7011 (terminal 70A).

The CLSF 7031 which obtained the ATM layer address (VCI/VPI) data fortransmitting the cell to the terminal 70A then outputs the cell attachedwith the VCI/VPI to the network.

Then, after the VCI/VPI conversion at the IWU 70G, the cell istransmitted to the IWU 70D. Similarly, the cell is transmitted to theCLSF 7011 via the IWU 70D. The CLSF 7011 which received the cell thenanalyzes the layer 3 address data of the datagram and transmits the cellto the terminal 70A.

Here, the cell transmission from the CLSF 7011 to the terminal 70A, thecell transmission from the CLSF-P 811 to the CLSF 7031, and the celltransmission from the CLSF 7031 to the IWU 70G may be made after all thecells belonging to the datagram are received by the CLSF 7011, theCLSF-P 811, or the CLSF 7031, or by the pipeline-like cell relayingafter the analysis of the layer 3 address data of the datagram.

3. Case Using VPI Routing

In this case, each sub-network has 8 bits sub-network identificationnumber. Within the defined network, the sub-network can be uniquelyidentified by this identification number. This identification number ofthe sub-network will be denoted as Net₁ below. For example, thesub-network 702 has the identification number N₇₀₂. Also, each IWU hasobtained the access address (VPI) for the connection-less communicationon its both sides. Moreover, each CLSF on the receiving side has alsoobtained the access address (VPI) for the connection-less communication,where this CLSF is for handling the cells arriving from the IWU whichcan be formed separately from the CLSF for handling the cells for theconnection-less communication coming from the terminals within its ownnetwork.

Here, the VCI/VPI field is coded according to the following manner.

(1) Cell from transmission source terminal to CLSF of its own network;

(1-1) VPI-F; access address of CLSF

(1-2) VCI-F1; address Net₁ of destination network or arbitrary

(1-3) VCI-F2; access address of itself

(2) Cell between CLSFs

(2-1) VPI-F; access address of next access element (IWU or destinationCLSF)

(2-2) VCI-F1; address Net_(destination) of destination network

(2-3) VCI-F2; address Net_(source) of network to which transmissionsource terminal belongs

(3) Cell from CLSF of network of destination terminal to destinationterminal

(3-1) VPI-F; access address of destination terminal

(3-2) VCI-F1; arbitrary setting possible

(3-3) VCI-F2; arbitrary setting possible

Here, "arbitrary setting possible" means it is possible to be set to anarbitrary value as long as it is set such that the cell received at thedestination terminal can be distinguished from a cell arriving from theother arbitrary access point for certain.

The above coding of the VCI/VPI can be applied in practice as follows.

(1) Sub-network containing transmission source terminal

First, the coding of the VCI/VPI field of the cell from the transmissionsource terminal to the CLSF can be done as follows. Namely, in thiscase, the VPI-F is an access address of the CLSF, while the VCI-F2 isdefined as the access address of the transmission source terminal. Asfor the VCI-F1, there are two cases of carrying out the addressresolution of the sub-network to which the destination terminal belongseither at the transmission source terminal or at the CLSF.

(a) In a case of carrying out the address resolution at the transmissionsource terminal, the identification number Net_(destination) of thedestination sub-network obtained by the transmission source terminal iswritten into the VCI-F1.

(b) In a case of carrying out the address resolution at the CLSF, theVCI-F1 can be set as an arbitrary value.

Next, the coding of the VCI/VPI field of the cell from the CLSF to theIWU can be done as follows. Namely, in this case, the VPI-F is an accessaddress of the IWU, and the VCI-F1 is set to be the identificationnumber Net_(destination) of the destination network, while the VCI-F2 isset to be the identification number Net_(source) of the network of thetransmission source terminal. Here, again, for the setting of theVCI-F1, there are two cases of carrying out the address resolution ofthe sub-network to which the destination terminal belongs either at thetransmission source terminal or at the CLSF.

(a) In a case of carrying out the address resolution at the transmissionsource terminal, the identification number Net_(destination) of thedestination sub-network obtained by the transmission source terminal iscopied into the VCI-F1.

(b) In a case of carrying out the address resolution at the CLSF, avalue obtained by the address resolution is written into the VCI-F1.

Here, in transmitting the cell from the CLSF to the IWU, theinterleaving between the cells belonging to the different datagrams butfor the same destination network is not allowed, although it is allowedas long as the destination network is different. Namely, a series ofcells belonging to one datagram are going to be transmittedcontinuously. In other words, it is possible to transmit the cell towardthe CLSF from each transmission source terminal at arbitrary timings,but the cells toward the IWU are transmitted from the CLSF datagram bydatagram. At the IWU, the value of the VPI-F is determined according tothe Net1_(destination) written in the VCI-F1 of the received cell.Namely, the IWU has a table registering the VPI-F value incorrespondence to the Net_(destination).

Also, the VCI-F1 and VCI-F2 are transmitted transparently, and therouting protocol for determining the VPI-F, i.e., the relaying targetsub-network, from the Net_(destination) is executed separately, and atable for each IWU is provided.

(2) Between IWUs

The cells are relayed according to the table of the IWU. Namely, theaccording to the Net_(destination) of the VCI-F1 of the received cell,the VPI-F for relaying the cell to the next IWU is written.

(3) The IWU to which the destination terminal belongs looks up theVCI-F1 of the received cell, and recognizes that the received cell isdestined to its own sub-network. The table provided in the IWU forsetting the VPI-F has the VPI value for transmitting the cell to theCLSF. The IWU sets this VPI to the VPI-F, and transmits the cell to theCLSF. At this point, the VCI-F1 and VCI-F2 are transmittedtransparently.

The CLSF which received the cell then carries out the re-assembling ofthe datagram according to the VCI data. Here, it is possible for thecells belonging to different datagrams to be interleaved, but thedatagrams can be reconstructed correctly even when the cells for thedatagrams are interleaved. The CLSF then analyzes the destinationaddress (layer 3 address) of the received datagram, and transmits thedatagram to the appropriate terminal.

the VPI-F of the cell transmitted from the CLSF to the terminal is theaccess address of the terminal. Next, the VCI field uses the VCI defined(possibly in plurality) for the cell of the connection-lesscommunication by the cell received by the terminal. This VCI is usuallyset up in advance between the CLSF and the terminal. In a case of havinga plurality of VCIs for connection-less communication, it is possiblefor the cell transmission from the CLSF to the terminal to be made inpipeline-like manner within a range of no conflict for theidentification numbers, i.e. the cells belonging to the differentdatagrams can be interleaved.

For example, when the VCI field format is defined by the communicationfrom the terminal to the CLSF, that is, when the VCI field format of thecommunications within the sub-network is uniform as such, the VCI-F2 ofthe cell received by the CLSF is directly set to the VCI-F2 of the cellto be transmitted to the terminal from the CLSF, and the VCI-F1 is setto be a value by which it is possible to identify this cell as the cellrelated to the connection-less communication, such that the datagram canbe processed completely in the pipeline-like manner.

Now, more concrete example of this case will be described in detail.

First, with reference to FIGS. 45 and 46, the datagram transmission fromthe terminal 70A to the terminal 70B will be described. In this case,three ATM connections are required, one formed by the connection 791from the terminal 70A to the CLSF 7011, another formed by theconnections 792 to 796 from the CLSF 7011 to the CLSF 7961, and stillanother formed by the connection 797 from the CLSF 7061 to the terminal70B.

The terminal 70A transmits the cell with the VPI-F having VPI₇₀₁₁, whilethe VCI-F2 has VPI_(70A) which is the access address of the terminal 70Aitself. Also, when the terminal 70A carries out the address resolutionof the address data (Net₇₀₆) of the sub-network to which the destinationterminal belongs by itself, this value Net₇₀₆ is written into theVCI-F1. On the other hand, when the CLSF 7011 carries out the addressresolution, the VCI-F1 can be set to an arbitrary value.

The CLSF 7011 transmits the cell with the VPI-F having VPI_(70D-1) whichis the access address of the IWU 70D, the VCI-F1 having Net₇₀₆ which isthe identification address of the network 706, and the VCI-F2 havingNet₇₀₁ which is the identification address of the network 701. Here, forthe VCI-F1, there are two cases of copying the value of the receivedcell directly (as the terminal analyzes Net₇₀₆) and analyzing andsetting Net₇₀₈ by the CLSF 7011.

The IWU 70D analyzes the VCI-F1 of the received cell, as well as thecorresponding VPI-F. Namely, the VPI for which the cell can betransmitted toward the IWU for transmitting to the network 706 isanalyzed according to the setting in the table. The VCI-F1 and VCI-F2are copied directly from these fields in the received cell. Namely, theVCI-F1 has Net₇₀₆ which is the network identification number of thesub-network 706 to which the destination terminal 70B belongs.Thereafter, similarly, the IWU 70D selects the appropriate VPI accordingto the data of Net₇₀₆ which is the data written in the VCI-F1 in thereceived cell, and transmits the cell to the IWU 70J.

The IWU 70J recognizes that the cell has reached the target networkaccording to the VCI-F1 data, and transmits the cell to the CLSF 7061.The datagram arrived to the CLSF 7061 is cell re-assembled once, and theATM connection is terminated. The CLSF 7061 analyzes the address data ofthe upper layer and recognizes that the destination of the datagram asthe terminal 70B. Then, the CLSF 7061 generates the cell and transmitsthe datagram to the terminal 70B. Here, the cell transmission to theterminal 70B can be done in the pipeline-like manner since the cellreception at the CLSf 7061 when there are sufficient amount of theconnection-less communication channels to the terminal 70B

Next, with reference to FIG. 47, the datagram transmission from theterminal 70A to the public network 708 is described. The terminal 70Atransmits the cell with VPI-F having VPI₇₈₁₁, while the VCI-F2 hasVPI_(70A) which is the access address of the terminal 70A itself. Also,when the terminal 70A carries out the address resolution of the addressdata (Net₇₀₈) of the sub-network to which the destination terminalbelongs by itself, this value Net₇₀₈ is written into the VCI-F1. On theother hand, when the CLSF 7011 carries out the address resolution, theVCI-F1 can be set to an arbitrary value.

The CLSF 7011 transmits to the IWU 70D the cell with the VPI-F havingVPI_(70D-1) which is the access address of the IWU 70D, the VCI-F1having Net₇₀₈ which is the identification address of the network 708,and the VCI-F2 having Net₇₀₁ which is the identification address of thenetwork 701. Here, for the VCI-F1, there are two cases of copying thevalue of the received cell directly (as the terminal analyzes Net₇₀₈)and analyzing and setting Net₇₀₈ by the CLSF 7011.

The IWU 70D analyzes the FCI-F1 of the received cell, as well as thecorresponding VPI-F. Namely, the VPI for which the cell can betransmitted toward the IWU for transmitting to the network 708 can beanalyzed according to the setting in the table. The VCI-F1 and VCI-F2are copied directly from these fields in the received cell. Namely, theVCI-F1 has Net₇₀₈ which is the network identification number of thesub-network 708 to which the destination terminal belongs. Thereafter,similarly, the IWU 70D selects the appropriate VPI according to the dataof Net₇₀₈ which is the data written in the VCI-V1 in the received cell,and transmits the cell to the IWU 70M.

The IWU 70M writes the VCI/VPI assigned to the ATM connection defined bythe public network 708 from the VCI data of the received cell, andtransmits the cell to the public network.

Datagram Delivery in Large Scale Network Architecture

In a case a number of defined sub-network is very large, the abovedescribed network architecture can be expanded by inter-networking aplurality of networks. Namely, in this scheme, neighboring networks(where each network is defined as a set of sub-networks as in the above)are regarded as a single sub-network of a large scale networkarchitecture.

FIG. 49 shows a configuration of such a large scale network architectureviewed from a network 861, in which there is another network 851 whichis inter-networking with the network 861. In reality, this networkarchitecture has a configuration as shown in FIG. 50, where the network851 actually comprises networks 862 and 863 and the public network 864to be regarded together as a single sub-network from the view of thenetwork 861. Thus, from the viewpoint of the network 861, the addressspace of the network 851 appears to contain those of the networks 862 to864 together.

FIG. 51 is a schematic diagram for this network system which shows onlythose elements relevant to the exemplary datagram transmissions from aterminal 87A in the network 961 to a terminal 87B in the network 863,and from the terminal 87A to a terminal 87C in the public network 864which will now be described in detail.

1. Scheme I

This is a scheme in which each terminal can transmit the cell to theexternal network directly. Namely, this is a scheme in which the CLSFwithin its own network is not utilized in the datagram transmission tothe terminal of the external network.

First, the datagram transmission from the terminal 87A to the terminal87B is carried out as follows.

In this case, four ATM connections 881 to 884 as shown in FIG. 52 arerequired, including an ATM connection 881 from the terminal 87A of thenetwork 861 to a CLSF 871 of the network 862, an ATM connection 882 fromthe CLSF 871 to a CLSF 872 of a sub-network in the network 863 connectedwith the network 862, an ATM connection 883 from the CLSF 872 to a CLSF873 of a sub-network in the network 863 connected with the terminal 87B,and an ATM connection 884 from the CLSF 873 to the terminal 87B.

The terminal 87A carries out the address resolution of the network layeraddress of the terminal 87B to recognize that the terminal 87B belongsto the network 851, while also obtains, the VCI/VPI data fortransmitting the cell to the CLSF 871 in the manner similar to thatdescribed above. Then, the datagram is transmitted from this terminal87A to the CLSF 871 through the ATM connection 881.

The datagram and the ATM connection are terminated once by the CLSF 871at which the layer 3 protocol processing for analyzing the network layeraddress of the datagram is carried out. As a result, it is recognizedthat the terminal 87B is present in the network 863, while also obtainsthe VCI/VPI data for transmitting the cell to the CLSF 872. Then, thedatagram is transmitted from this CLSF 871 to the CLSF 872 through theATM connection 882.

The datagram and the ATM connection are terminated again by the CLSF 872at which the layer 3 protocol processing for analyzing the network layeraddress of the datagram is carried out, while the VCI/VPI data fortransmitting the cell to the CLSF 873 is also obtained. Then, thedatagram is transmitted from this CLSF 872 to the CLSF 873 through theATM connection 883.

The CLSF 873 which received the datagram then analyzes the network layeraddress of the datagram to analyze the access address of the terminal87B. Then, the cell with the appropriate VCI/VPI attached is transmittedto the terminal 87B through the ATM connection 884.

In this manner, the termination of the ATM connection and the networklayer protocol processing are carried out three times to transmit thedatagram from the terminal 87A to the terminal 87B.

Next, the datagram transmission from the terminal 87A to the terminal87C is carried out as follows.

In this case, two ATM connections 891 and 892 as shown in FIG. 53 arerequired, including an ATM connection 891 from the terminal 87A of thenetwork 861 to the CLSF 871 of the network 862, and an ATM connectionfrom the CLSF 871 to the terminal 87C in the public network 864.

The terminal 87A carries out the address resolution of the network layeraddress of the terminal 87C to recognize that the terminal 87C belongsto the network 851, while also obtains the VCI/VPI data for transmittingthe cell to the CLSF 871 in the manner similar to that described above.Then, the datagram is transmitted from this terminal 87A to the CLSF 871through the ATM connection 891.

The datagram and the ATM connection are terminated once by the CLSF 871at which the layer 3 protocol processing for analyzing the network layeraddress of the datagram is carried out. As a result, it is recognizedthat the terminal 87C is present in the network 864, while also obtainsthe VCI/VPI data for transmitting the cell to the network VCI/VPI. Then,the datagram is transmitted from this CLSF 871 to the IWU providedbetween the networks 862 and 864 through the ATM connection 892.

In this manner, the termination of the ATM connection and the networklayer protocol processing are carried out once to transmit the datagramfrom the terminal 87A to the network 864 containing the terminal 87C.Here, the datagram transmission target from the CLSF 871 may be the CLSFprovided within the public network 864 if desired. In such a case, thetermination of the ATM connection and the network layer protocolprocessing are carried out more than once.

2. Scheme II

This is a scheme in which each terminal cannot transmit the cell to theexternal network directly. Namely, this is a scheme in which the CLSFwithin its own network is utilized in the datagram transmission to theterminal of the external network.

First, the datagram transmission from the terminal 87A to the terminal87B is carried out as follows.

In this case, five ATM connections 901 to 905 as shown in FIG. 54 arerequired, including an ATM connection 901 from the terminal 87A to aCLSF 874 within the network 861, an ATM connection 902 from the CLSF 874to a CLSF 871 of the network 862, an ATM connection 903 from the CLSF871 to a CLSF 872 of a sub-network in the network 863 connected with thenetwork 862, an ATM connection 904 from the CLSF 872 to a CLSF 873 of asub-network in the network 863 connected with the terminal 87B, and anATM connection 905 from the CLSF 873 to the terminal 87B.

The terminal 87A carries out the address resolution of the network layeraddress of the terminal 87B to recognize that the terminal 87B belongsto the network 851 (or that the cell is to be transmitted to the CLSF874), while also obtains the VCI/VPI data for transmitting the cell tothe CLSF 874 in the manner similar to that described above. Then, thecell is transmitted from this terminal 87A to the CLSF 874 through theATM connection 901.

The CLSF 874 which received this cell recognizes that there is a need totransmits the cell to the network 862 (or CLSF 871) according to theanalysis of the network layer address of the datagram obtained either bythe CLSF 874 itself or by the terminal 87A, while also obtains theVCi/VPI data for transmitting the cell to the CLSF 871. Them, thedatagram is transmitted from this CLSF 874 to the CLSF 871 through theATM connection 902.

The datagram and the ATM connection are terminated once by the CLSF 871at which the layer 3 protocol processing for analyzing the network layeraddress of the datagram is carried out. As a result, it is recognizedthat the terminal 87B is present in the network 863, while also obtainsthe VCI/VPI data for transmitting the cell to the CLSF 872. Then, thedatagram is transmitted from this CLSF 871 to the CLSF 872 through theATM connection 903.

The datagram and the ATM connection are terminated again by the CLSF 872at which the layer 3 protocol processing for analyzing the network layeraddress of the datagram is carried out, while the VCI/VPI data fortransmitting the cell to the CLSF 873 is also obtained. Then, thedatagram is transmitted from this CLSF 872 to the CLSF 873 through theATM connection 904.

The CLSF 873 which received the datagram then analyzes the network layeraddress of the datagram to analyze the access address of the terminal87B. Then, the cell with the appropriate VCI/VPI attached is transmittedto the terminal 87B through the ATM connection 905.

In this manner, the termination of the ATM connection and the networklayer protocol processing are carried out four (or three) times totransmit the datagram from the terminal 87A to the terminal 87B.

Next, the datagram transmission from the terminal 87A to the terminal87C is carried out as follows.

In this case, three ATM connections 911 to 913 as shown in FIG. 55 arerequired, including an ATM connection 911 from the terminal 87A to aCLSF 874 within the network 861, an ATM connection 912 from the CLSF 874to a CLSF 871 of the network 862, and an ATM connection 913 from theCLSF 871 to the terminal 87C in the public network 864.

The terminal 87A carries out the address resolution of the network layeraddress of the terminal 87C to recognize that the terminal 87C belongsto the network 851 (or that the cell is to be transmitted to the CLSF874), while also obtains the VCI/VPI data for transmitting the cell tothe CLSF 874 in the manner similar to that described above. Then, thedatagram is transmitted from this terminal 87A to the CLSF 874 throughthe ATM connection 911.

The CLSF 874 which received this cell recognizes that there is a need totransmits the cell to the network 862 (or CLSF 871) according to theanalysis of the network layer address of the datagram obtained either bythe CLSF 874 itself or by the terminal 87A, while also obtains theVC1/VPI data for transmitting the cell to the CLSF 871. Them, thedatagram is transmitted from this CLSF 874 to the CLSF 871 through theATM connection 912.

The datagram and the ATM connection are terminated once by the CLSF 871at which the layer 3 protocol processing for analyzing the network layeraddress of the datagram is carried out. As a result, it is recognizedthat the terminal 87C is present in the network 864, while also obtainsthe VCI/VPI. Then, the datagram is transmitted from this CLSF 871 to theIWU provided between the networks 862 and 864 through the ATM connection913.

In this manner, the termination of the ATM connection and the networklayer protocol processing are carried out twice (or once) to transmitthe datagram from the terminal 87A to the network 864 containing theterminal 87C. Here, the datagram transmission target from the CLSF 871may be the CLSF provided within the public network 864 if desired. Insuch a case, the termination of the ATM connection and the network layerprotocol processing are carried out more than twice.

Application to Modified Network Layer Topology

Now, the embodiment in which the above described ATM communicationsystem according to thepresent invention is applied for a case ofrealizing a modified network layer topology independent from thetopology of the physical network will be described in detail.

FIG. 56 shows a configuration of the ATM communication system in thisembodiment, which comprises: a first ATM-LAN 1101 containing M terminals151 to 15M and a second ATM-LAN 1102 containing N terminals 121 to 12Nwhich are inter-networking through an IWU 1111. Here, the first ATM-LAN1101 has a CLSF 1112 for carrying out the processing for realizing theconnection-less communication, whereas the second ATM-LAN 1102 has noCLSF.

The CLSF 1112 of the first ATM-LAN 1101 not only supports the datagramtransmission among the terminals 151 to 15M of the first ATM-LAN 1101,but also the datagram transmission among the terminals among theterminals 121 to 12N of the second ATM-LAN 1102 as well, and to thisend, at the network layer, the CLSF 1112 is given in advance the firstaddress data indicating that it belongs to the first ATM-LAN 1101 inwhich it is physically located, as well as the second address dataindicating that it also belongs to the second ATM-LAN 1102. These firstand second address data will be referred hereafter as the network IDs.

The IWU 1111 inter-networking the first and second ATM-LANs 1101 and1102 has a schematic configuration shown in FIG. 57 which includes aheader conversion table 1172 and a header conversion unit 1173, wherethe header conversion table 1172 registers the relationship between theheader (or the connection identifier, i.e., VCI/VPI) of the input cellfrom the ATM connection, and the header of the output cell to the ATMconnection, while the header conversion unit 1173 converts the header ofthe input cell by looking up the header conversion table 172 andattaches the converted header to the output cell.

In addition, the first and second ATM-LANs 1101 and 1102 have call setup units 1113 and 1114, respectively. The call set up unit 1113 of thefirst ATM-LAN 1101 sets up the ATM connections 141 to 14N between theIWU 1111 and the CLSF 1112, as well as the ATM connections 161 to 16Mbetween the CLSF 1112 and the terminals 151 to 15M belonging to thefirst ATM-LAN 1101, while the call set up unit 1114 of the secondATM-LAN 1102 sets up the ATM connections 131 to 13N between the IWU 1111and the terminals 121 to 12N belonging to the second ATM-LAN 1102.

It is to be noted that the configuration shown in FIG. 56 represents aminimum unit of the communication system according to this embodiment,and a larger scale configuration can be constructed by inter-networkinga number of such minimum units through IWUs.

Now, the operation of this embodiment will be described for an exemplarycase of realizing the connection-less communication from the terminal1121 in the second ATM-LAN 1102 by using the CLSF 1112 in the firstATM-LAN 1101. In this case, the second ATM-LAN 1102 containing thisterminal 1121 has no CLSF for realizing the connection-lesscommunication itself, so that the CLSF 1112 of the first ATM-LAN 1101 ismade to be also available for the terminal 1121 of the second ATM-LAN1102 as described below.

Namely, in this case, the operation proceeds as outlined in the flowchart of FIG. 58 as follows.

First, at the step S11, in order to set up the ATM connection betweenthe terminal 1121 and the CLSF 1112, the call set up unit 1114 sets upthe first ATM connection 1131 between the terminal 1121 and the IWU1111. Then, at the step S12, the call set up unit 1114 makes the set uprequest for the ATM connection between the IWU 1111 and the CLSF 1112 tothe call set up unit 1113 of the first ATM-LAN 1101, as the first andsecond ATM-LANs 1101 and 1102 are independent networks. Then, inresponse to this set up request, at the step S13, the call set up 1113sets up the ATM connection 1141 between the IWU 1111 and the CLSF 1112.

Here, the method of making the ATM connection set up request from thecall set up unit 1114 to the call set up unit 1113 can be either one of:(1) providing an ATM connection 1171 between the call set up unit 1113and the call set up unit 1114 in advance, and the set up request is madedirectly through this ATM connection 1171, or (2) terminating the set uprequest from the call set up unit 1114 once at the IWU 1111 and thenrelaying it to the call set up unit 1113 from the IWU 1111.

When the ATM connections 1131 and 1141 are set up in this manner, nextat the step S14, the IWU 1111 connects these ATM connections 1131 and1141 at the ATM layer. In this case, the header conversion table 1172has registered entries such that: (a) the header of the cell arrivingfrom the ATM connection 1131 is changed to the connection identifier(VCI/VPI) indicating the ATM connection 1141, and (b) the header of thecell arriving from the ATM connection 1141 is changed to the connectionidentifier (VCI/VPI) indicating the ATM connection 1131. The headerconversion unit 1173 converts the header of the arriving cell by lookingup this header conversion table 1172, and transmits the cell arrivingfrom the ATM connection 1131 to the ATM connection 1141, and the cellarriving from the ATM connection 1141 to the ATM connection 1131. Inthis manner, the ATM connections 1131 and 1141 are connected at the ATMlayer by the IWU 1111.

Next, at the step S15, the datagram transmission between the terminal1121 and the CLSF 1112 is carried out through the ATM connections 1131and 1141 connected at the ATM layer. In this case, using the connectionbetween the ATM connections 1131 and 1141, the datagram transmission canbe realized by simply carrying out the header conversion processing atthe ATM layer in the IWU 111, for both of the datagram to be deliveredfrom the terminal 1121 to the CLSF 1112 as well as the ATM cellassembled datagram to be delivered from the CLSF 1112 to the terminal1121.

Here, the flow of the cell assembled datagram between the terminal 1121and the CLSF 1112 is as follows.

First, in a case (1) of the datagram transmission from the terminal 1121to the CLSF 1112, (1-1): the cell assembled datagram is transmitted tothe IWU 1111 from the terminal 1121 through the ATM connection 1131.(1-2): the IWU 1111 relays the cell arrived from the ATM connection 1131to the ATM connection 1141 by looking up the header conversion table1172, and(1-3): the CLSF 1112 receives the cell assembled datagramarriving from the ATM connection 1141.

On the contrary, in a case (2) of the datagram transmission from theCLSF 1112 to the terminal 1121. (2-1): the cell assembled datagram istransmitted to the IWU 1111 from the CLSF 1112 through the ATMconnection 1141, (2-2): the IWU 1111 relays the cell arrived from theATM connection 1141 to the ATM connection 1131 by looking up the headerconversion table 1172, and(2-3): the terminal 1121 receives the cellassembled datagram arriving from the ATM connection 1131.

In this manner, it is possible in this embodiment to transmit the celldirectly at the network layer between the terminal 1121 and the CLSF1112. The cell transmission between the CLSF 1112 and any of the otherterminals 1122 to 112N through the ATM connections 1132 to 113N and 1142to 114N can also be realized similarly.

Next, an exemplary processing at the network layer in this embodimentwill be described with reference to FIG. 59 which indicates the physicalregions of the ATM communication system of FIG. 56 along with thelogical regions at the network level and the logical connection statesamong these logical reasons.

Here, between the terminals 1121 to 112N of the second ATM-LAN 1102 andthe CLSF 1112 of the first ATM-LAN 1101, the ATM connections 1131 to113N and 1141 to 114N are provided as in FIG. 56, such that the datagramtransmission can be realized in forms of the ATM cells between the CLSF1112 and the terminals 1121 to 112N. In addition, the ATM connections1181 to 116M are also provided between the CLSF 1112 and the terminals1151 to 115M of the first ATM-LAN 1101.

The CLSF 1112 has a datagram processing unit 1201 and an ATM layerprocessing unit 1211, and the terminals 1121 to 112N have a dataprocessing unit 1202 and an ATM layer processing unit 1212. The datagramprocessing unit 1201 operates according to the network ID indicatingthat the CLSF 1112 belongs to the second ATM-LAN 1102 with respect tothe input and output through the ATM connections 1141 to 114N, oraccording to the network ID indicating that the CLSF 1112 belongs to thefirst ATM-LAN 1101 with respect to the input and output through the ATMconnections 1161 to 116M.

In this case, the datagram outputted from the datagram processing unit1202 within the terminals 1121 to 112N is cell assembled at the ATMlayer processing unit 1212 within the terminals 1121 to 112N, andreaches to the CLSF 1112 without being re-constructed into the datagramform before being cell disassembled at the ATM layer processing unit1211 within the CLSF 1112. Similarly, the datagram outputted from thedatagram processing unit 1202 within the CLSF 1112 is cell assembled atthe ATM layer processing unit 1211 within the terminals 1121 to 112N,and reaches to the terminals 1121 to 112N without being re-constructedinto the datagram form before being cell disassembled at the ATM layerprocessing unit 1212 within the terminals 1121 to 112N.

In this manner, the direct delivery (i.e., the delivery without lookingup the network layer data in a middle) of the datagram at the networklayer between the terminals 1121 to 112N and the CLSF 1112 is supported,so that there is no need for the datagram processing unit 1202 withinthe terminals 1121 to 112N to recognize the fact that the CLSF 1112 isactually located in the physically separated first ATM-LAN 1101.Similarly, there is no need for the datagram processing unit 1201 withinthe CLSF 1112 to recognize the fact that the terminals 1121 to 112N areactually located in the physically separated second ATM-LAN 1102.

In addition, at the network layer level, the CLSF 1112 and the terminals1121 to 112N share identical address data, while the CLSF 1112 and theterminals 151 to 15M also share identical address data.

Here, the network layer address is given in a form shown in FIG. 60 inwhich the network ID described above is multiplexed with the terminal IDindicating the specific address of each terminal. The CLSF 1112 has twosets of network IDs in correspondence to the first and second ATM-LANs1101 and 1102, which are shared with the terminals within the respectiveATM-LANs 1101 and 1102. It is noted that the order of the network ID andthe terminal ID in the network layer address may be reversed from thatshown in FIG. 60 if desired.

It is also noted that the scheme for providing two sets of network IDsto be CLSF 1112 with respect to the first and second ATM-LANs 1101 and1102 is adopted in this embodiment because of the easiness of itsimplementation, but it is also possible to adopt the scheme in which theCLSF 1112 (i.e., the network address belonging to the first ATM-LAN 1101which has the CLSF 1112) can be made to appear as if it is virtuallybelonging to the second ATM-LAN 1102 by operating the CLSF 1112 and theterminals 1121 to 112N accordingly, at a time of mapping the ATM layerand executing the network layer protocol.

By setting up the network layer address in this manner, it becomespossible at the network layer to handle the CLSF 1112 within the firstATM-LAN 1101 as if it is belonging to the second ATM-LAN 1102 as wellsuch that the terminals 1121 and 112N and the CLSF 1112 can be treatedas if they are belonging to the same network. Namely, the logical regionof the second ATM-LAN 1102 at the network layer includes the physicalregion 1221 of the second ATM-LAN 1102 as well as the ATM connections1141 to 114N and the CLSF 1112, as indicated by the hatched area in FIG.59. In this case, the logical region of the first ATM-LAN 1101 at thenetwork layer is going to be the physical region of the first ATM-LAN1101 from which the ATM connections 1141 to 114N are excluded, so thatthe CLSF 1112 logically belongs to both of the first and second ATM-LANs1101 and 1102. Here, however, it is to be noted that these logicalregions are valid only for the network layer of the connection-lesscommunication (datagram communication), and the logical regions for theATM layer and the logical regions for the network layer of theconnection oriented communication are generally different from thelogical regions in the connection-less communication.

In this manner, for the connection-less communication, the logicalregions at the network layer which is different from the physicalregions of the ATM-LANs to realize the network layer topology 1231 shownin FIG. 59 in which the CLSF 1112 can be treated as a router (or agate-way).

Therefore, when the routing protocol is executed among the CLSFs, theCLSF 1112 logically appears at the network layer as if it is located inthe ATM-LAN which physically contains no CLSF. Consequently, even whenthe already existing routing protocol is executed as it is, the routeselected by the routing protocol and the route passing through the CLSF1112 coincides, so that the consistent connection-less communication canbe realized. In this case, it suffices for the IWU 1111 to pass therouting data for the connection-less communication, so that the IWU 1111can be totally free from the routing in the connection-lesscommunication.

FIG. 61 shows another network topology which can be handled in themanner similar to that described above. In this FIG. 61, three ATM-LANs1301 to 1303 are inter-networking through IWUs (not shown) with the CLSF1331, where the CLSF 1331 physically belongs to the ATM-LAN 1301. Inthis case, by setting up the network layer address in the manner similarto that described above, it becomes possible to make the CLSF 1331 toappear as if it is also logically belonging to the ATM-LANs 1302 and1303. Consequently, it becomes possible to realize the logical networklayer topology in which the ATM-LANs 1301 to 1303 are inter-networkingwith the CLSF 1331 as a router (or a gate-way).

Now, the manner of executing the routing protocol among the CLSFs inmore general case of this embodiment will be described with referencesto FIGS. 62 and 63. Here, the network configuration of FIG. 56 isexpanded such that seven ATM-LANs 1401 to 1407 are inter-networkingthrough six IWUs 1411 to 1416 as shown in FIG. 62, where only theATM-LANs 1401, 1403, and 1406 contain the CLSFs 1421 to 1423,respectively, while the other ATM-LANs 1402, 1404, 1405, and 1407 haveno CLSF physically. Here, however, by setting the network layeraddresses as in the above, at the network layer level, the CLSF 1421logically belongs to the ATM-LAN 1402, the CLSF 1422 logically belongsto the ATM-LAN 1404 and 1405, and the CLSF 1423 logically belongs to theATM-LAN 1407.

In this case, the ATM connection 1441 is set up between the CLSFs 1421and 1422, while the IWU 1412 which in inter-networking the ATM-LANs 1401and 1403 has a function for passing the data as it is with respect tothe data transmission including that of the routing data between theCLSFs 1421 and 1422.

Similarly, the ATM connection 1442 is set up between the CLSFs 1421 and1423, while the IWU 1415 which is inter-networking the ATM-LANs 1401 and1406 has a function for passing the data as it is with respect to thedata transmission including that of the routing data between the CLSFs1421 and 1423.

Here, a scheme for setting up the ATM connections 1441 and 1442 can beeither one of a scheme for setting up one ATM connection for thetransmission of the datagram as well as the control data such as therouting data between the CLSFs, or a scheme for setting up different ATMconnections for the datagram transmission and the transmission of thecontrol data such as the routing data between the CLSFs.

As a concrete example, a case of executing the routing protocol calledRIP (Routing Information Protocol) which has been conventionally used inthe internet, in the configuration of FIG. 63 will be described. ThisRIP is a routing protocol of a vector distance type in which the routingis controlled to minimize the number of routes to be used according to ahop number for the routers to be passed in the middle. In thisembodiment, the CLSF can be logically treated as a router as describedabove, so that this RIP can be executed to carry out the activeoperation (i.e., the operation to notify the routing data to the otherrouters) by each CLSF.

Namely, the CLSF 1422 notifies the CLSF 1421 that "ATM-LANs 1403, 1404,and 1405 can be reached by one hop" as the ATM-LANs 1403, 1404, and 1405are the networks to which the CLSF 1422 can deliver the datagramdirectly. The CLSF 1423 similarly notifies the CLSF 1421 that "ATM-LANs1406 and 1407 can be reached by one hop" as the ATM-LANs 1406 and 1407are the networks to which the CLSF 1423 can deliver the datagramdirectly. Also, the CLSF 1421 notifies the CLSFs 1422 and 1423 that"ATM-LANs 1401 and 1402 can be reached by one hop" as the ATM-LANs 1401and 1402 are the networks to which the CLSF 1421 can deliver thedatagram directly.

In addition, when the routing data from the CLSFs 1422 and 1423 havereached to the CLSF 1421 at this point, the CLSF 1421 notifies the CLSFs1422 and 1423 that "ATM-LANs 1403 to 1407 can be reached by two hops" byadding one hop to the routing data reached from the CLSFs 1422 and 1423.

Here, the CLSFs 1421 to 1423 have associated routing tables 1451 to 1453which indicate the correspondence relationship between the datagramdestination and the delivering target ATM-LAN (transmission target) in acase of operating the RIP at the CLSFs 1421 to 1423. Thus, each of theCLSFs 1421 to 1423 calculates the route from the received routing dataand selects the shortest route in order to determine the deliveringtarget for the datagram. In the routing tables 1451 to 1451 to 1453, theentry with "direct delivery" registered as the delivery target indicatesthe network which can be reached by 0 hop.

For example, in a case of the CLSF 1421, the routing table 1451 isconstructed according to the routing data indicating that: (1) theATM-LANs 1401 and 1402 can be delivered directly by itself, (2) theATM-LANs 1403, 1404, and 1405 can be reached by one hop from the CLSF1422, and (3) the ATM-LANs 1406 and 1407 can be reached by one hop fromthe CLSF 1423.

For example, in a case of the CLSF 1422, the routing table 1452 isconstructed according to the routing data indicating that: (1) theATM-LANs 1403, 1404, and 1405 can be delivered directly by itself, (2)the ATM-LANs 1401 and 1402 can be reached by one hop from the CLSF 1421,and (3) the ATM-LANs 1403, 1404, 1405, 1406, and 1407 can be reached bytwo hops from the CLSF 1421. Here, for the ATM-LANs 1403, 1404, and1405, the directly delivering route is going to be the shortest route,so that the directly delivering route is registered in the routing table1452.

For example, in a case of the CLSF 1423, the routing table 1453 isconstructed according to the routing data indicating that: (1) theATM-LANs 1406 and 1407 can be delivered directly by itself, (2) theATM-LANs 1401 and 1402 can be reached by one hop from the CLSF 1421, and(3) the ATM-LANs 1403, 1404, 1405, 1406, and 1407 can be reached by twohops from the CLSF 1421. Here, for the ATM-LANs 1406 and 1407, thedirectly delivering route is going to be the shortest route, so that thedirectly delivering route is registered in the routing table 1453.

The procedure for carrying out the connection-less communication overthe ATM-LANs using these routing tables 1451 to 1453 will be describedwith reference to FIG. 63, for an exemplary case of the datagramtransmission from the terminal 1461 belonging to the ATM-LAN 1404 to theterminal 1462 belonging to the ATM-LAN 1407. In this case, by the callset up units provided in the networks, the ATM connection 1443 betweenthe terminal 1461 and the CLSF 1422 and the ATM connection 1444 betweenthe terminal 1462 and the CLSF 1423 are set up in advance in the mannerdescribed above.

At the terminal 1461, the data from the application is processed toobtain the datagram to be transmitted to the terminal 1462 at thedatagram processing unit 4611, and the cell is assembled from thedatagram at the cell assembling unit 4612, and the assembled cell isoutputted from the cell transmission unit 4613. The cell outputted fromthe terminal 1461 then reaches to the CLSF 1422 through the ATMconnection 1443.

At the CLSF 1422, the arrived cell is received at the cell receptionunit 4221, and the cell is disassembled at the cell disassembling unit4222, and the datagram is reproduced at the datagram processing unit4223, and then the network ID of the transmission target given in a formshown in FIG. 60 is transmitted to the routing protocol execution unit4224. At the routing protocol execution unit 4224, according to thisnetwork ID, it can be recognized that the terminal 1462 belongs to theATM-LAN 1407, so that the routing table 1452 is looked up to determinethe delivery target as the CLSF 1421. Then, the ATM address search unit4225 looks up the ATM address table 1472 according to the network ID ofthe CLSF 1421 to obtain the VCI/VPI value appropriate for the datagramtransmission to the CLSF 1421, and transmits the obtained VCI/VPI valueto the cell assembling unit 4226. The cell assembling unit 4226 thenassembles the cell from the payload including the datagram transmittedfrom the datagram processing unit 4223 according to the routing datasupplied from the routing protocol execution unit 4224 and the VCI/VPIvalue supplied from the ATM address search unit 4225, and the assembledcell is outputted from the cell transmission unit 4227.

The cell outputted from the CLSF 1422 is then transmitted to the CLSF1421 through the ATM connection 1441 and the IWU 1412 provided betweenthe CLSFs 1421 and 1422. The IWU 1412 only applies the ATM layerprocessing (which is the processing requiring no cell re-assembling) tothe cell from the CLSF 1422 and pass it to the CLSF 1421.

At the CLSF 1421, the processing similar to that carried out at the CLSF1422 is carried out, and the routing table 1451 is looked up todetermine the delivery target as the CLSF 1423. Then, the cell isassembled by obtaining the appropriate VCI/VPI value from the ATMaddress table 1471, and the assembled cell is outputted to the CLSF1423. The cell outputted from the CLSF 1421 is then transmitted to theCLSF 1423 through the ATM connection 1442 and the IWU 1415 providedbetween the CLSFs 1421 and 1423.

At the CLSF 1423, the processing similar to that carried out at the CLSF1421 is carried out, and the routing table 1453 is looked up torecognize that the delivery target can be delivered directly (as theCLSF 1423 itself is registered as the delivery target by the RIP), i.e.,the destination is the terminal 1462 which is directly connected by theATM connection from the CLSF 1423. Then, by looking up the ATM addresstable 1473, the ATM connection 1444 is registered in correspondence tothe terminal 1462, so that the CLSF 1423 outputs the cell assembled fromthe datagram to the terminal 1462 through the ATM connection 1444. Thecell outputted from the CLSF 1423 is then transmitted to the terminal1462 through the ATM connection 1444 and the IWU 1416 provided betweenthe CLSF 1423 and the terminal 1462.

At the terminal 1462, the cell arriving through the ATM connection 1444is received at the cell reception unit 4621, the cell is disassembled atthe cell disassembling unit 4622, and the datagram is reconstructed atthe datagram processing unit 4623, and the re-constructed datagram istransmitted to the application.

In this manner, the datagram transmission over the ATM-LANs using therouting protocol can be realized. Here, as for the routing protocolwhich is operated among the CLSFs, the the routing data is cellassembled by the routing protocol execution unit within the CLSF, whilealso supplied to the other CLSFs connected though the ATM connections,and the CLSF which received the routing data re-constructs the datagramat the datagram processing unit within the CLSF, and transmits therouting data to the routing protocol execution unit.

It is to be noted that the datagram transmission in the single directionof the terminal 1461 → CLSF 1422 → CLSF 1421 → CLSF 1423 → terminal 1462has been described above as an example, but this embodiment is equallyvalid for the datagram transmission in the opposite direction as well inthe substantially similar manner.

Next, a scheme for transmitting the routing data from the CLSF to theATM-LAN in which the CLSF is absent will be described with reference toFIG. 64, the ATM-LANs 1501 and 1502 are inter-networking through the IWU1511, where the ATM-LAN 1501 has the CLSF 1521 while the ATM-LAN 1502has no CLSF. Here, the CLSF 1521 is logically connected with the ATM-LAN1502 as in the above such that the CLSF 1521 can appear to be logicallypresent in the ATM-LAN 1502 as well. Although not shown in FIG. 64, theATM-LANs 1501 and 1502 are also connected with the other ATM-LANsthrough the IWUs, and the CLSF 1521 is executing the routing protocol.

The CLSF 1521 transmits the routing data to the device 1531 whichrequires the routing data within the ATM-LAN 1501 in which the CLSF 1521is contained as well as to the device 1532 which requires the routingdata within the ATM-LAN 1502 which logically belongs to the same networkas the ATM-LAN 1501. Here, the devices 1531 and 1532 which require therouting data can be any of the datagram terminals, the addressresolution server (ARS) for setting a correspondence between the networkID and the VCI/VPI, and the IWU.

In this case, the routing data to the device 531 can be delivered fromthe CLSF 1521 as it is within the ATM-LAN 1501 as this device 1531 islocated within the ATM-LAN 1501 in which the CLSF 1521 is also located.

On the other hand, in order to deliver the routing data from the CLSF1521 to the ATM-LAN 1502, it suffices for the IWU 1511 provided betweenthe CLSF 1521 and the ATM-LAN 1502 to pass the routing data transmittedfrom the CLSF 1521 to the device 532 as it is, because there is no needto process the routing data as the CLSF 1521 also belongs to the ATM-LAN1502 at the network layer. To this end, there is a need for the CLSF1521 and the device 1532 to be connected at the ATM layer even when theIWU 151 is located therebetween. This can be achieved by using the ATMconnection for the logical connection as in the above, or by setting upthe separate ATM connection for the routing data transmission.

As an example, a case of using the RIP described above will bedescribed. In this case, as shown in FIG. 64, the CLSF 1521 has therouting table 1541, and the devices 1531 and 1532 which require therouting data are executing the RIP to make the passive operation (i.e.,the operation in which the routing data is received and processed, butnot transmitted to the others). In this case, according to the routingdata received from the CLSF 1521, the device 1531 within the ATM-LAN1501 can construct the routing table 1551, while the device 1532 withinthe ATM-LAN 1502 can construct the routing table 1552.

When these routing tables 1551 and 1552 are constructed, they canfunction as means for selecting the direct datagram transmission withoutpassing through CLSF when the destination terminal is located within thesame ATM-LAN.

Here, the datagram cell processing time at the IWU 1511 at a time ofexecuting the RIP is sufficiently short compared with the time requiredfor the datagram reconstruction and the cell assembling at the CLSF1521, so that it is possible to disregard the passing through the IWU1411 from the hop count.

Now, FIG. 65 shows another configuration of the ATM communication systemin this embodiment, which comprises: a first ATM-LAN 1601 containing Nterminals 1631 to 163N, a second ATM-LAN 1602 containing N terminals1641 to 164N, and a third ATM-LAN 1603 containing N terminals 1651 to165N, where the first and second ATM-LANs 1601 and 1602 areinter-networking through an IWU 1611, and the second and this ATM-LANs1602 and 1603 are inter-networking through an IWU 1612. Here, the firstATM-LAN 1601 has a CLSF 1613 for carrying out the processing forrealizing the connection-less communication, whereas the second andthird ATM-LANs 1602 and 1603 have no CLSF.

In addition, the ATM-LANs 1601, 1602, and 1603 have call set up units1616, 1615, and 1614, respectively. The call set up unit 1113 of thefirst ATM-LAN 1101 sets up the ATM connections 141 to 14N between theIWU 1111 and the CLSF 1112, as well as the ATM connections 161 to 16Mbetween the CLSF 1112 and the terminals 151 to 15M belonging to thefirst ATM-LAN 1101, while the call set up unit 1114 of the secondATM-LAN 1102 sets up the ATM connections 131 to 13N between the IWU 1111and the terminals 121 to 12N belonging to the second ATM-LAN 1102.

The CLSF 1613 of the first ATM-LAN 1601 not only supports the datagramtransmission among the terminals (not shown) of the first ATM-LAN 1601,but also the datagram transmission among the terminals among theterminals (not shown) of the second ATM-LAN 1602 and the datagramtransmission among the terminals among the terminals 1621 to 162N aswell, and to this end, at the network layer, the CLSF 1112 is given notonly the first network ID indicating that it belongs to the firstATM-LAN 1601 in which it is physically located, as well as the secondand third network IDs indicating that it also belongs to the second andthird ATM-LANs 1602 and 1603. In this manner, the CLSF 1613 can be madeto appear as if it is in the second ATM-LAN 1602 or the third ATM-LAN1603.

In this case, the connection-less communication (datagram transmission)between the CLSF 1613 and the terminals 1621 to 162N in the ATM-LAN 1603requires the passing through two IWUs 1611 and 1612. This operation willnow be described.

Here, as described above, the ATM-LAN 1603 has no CLSF, so that it isnecessary to make the CLSF 1613 in the ATM-LAN 1601 to be available tothe terminals in the ATM-LAN 1603. In the following, an exemplary caseof the datagram transmission from the terminal 1621 in the ATM-LAN 1603will be described.

In this case, the operation proceeds according to the flow chart of FIG.66 as follows.

First, at the step S21, in order to set up the ATM connection betweenthe terminal 1621 and the CLSF 1613, the call set up unit 1614 sets upthe ATM connection 1631 between the terminal 621 and the IWU 1612. Then,at the step S22, the call set up unit 1614 makes the ATM connection setup request to the call set up unit 1615 in the ATM-LAN 1602 which is theindependent network from the ATM-LAN 1603 to which the call set up unit1614 belongs.

Here, however, the ATM-LAN 1602 also has no CLSF, so that the call setup request unit 1615 which received the ATM connection set up requestfrom the call set up unit 1614 sets up the ATM connection 1641 betweenthe IWU 1612 and the IWU 1611 at the step S23, and makes the call set uprequest for the CLSF 1613 to the call set up unit 1616 in theneighboring ATM-LAN 1601 at the step S24. Then, the call set up unit1616 sets up the ATM connection 1651 between the IWU 1611 and the CLSF1613 in response to the received call set up request at the step S25.

When the ATM connections 1631, 1641, and 1651 are set up in this manner,next at the step S26, the IWU 1612 connects the ATM connections 1631 and1641 at the ATM layer, while the IWU 1611 connects the ATM connections1641 and the 1651 at the ATM layer in the same manner as in the above.

Then, at the step S27, the datagram transmission between the terminal1621 and the CLSF 1613 is carried out through the ATM connections 1631,1641, and 1651 connected at the ATM layer in terms of the ATM cells.

The similar procedure can also be followed for the other terminals 1622to 162N, to set up the ATM connections 1632 to 163N 1642 to 164N, and1652 to 165N, connect the ATM connections 1632 to 163N and the ATMconnections 1642 to 164N at the IWU 1612, connect the ATM connections1642 to 164N and the ATM connections 1652 to 165N at the IWU 1611, andcarry out the datagram transmissions in terms of the ATM cells betweenthe terminals 1622 to 162N to the CLSF 1613.

By setting up the ATM connections in this manner, it becomes possible tocarry out the direct datagram delivery at the network layer between theterminals 1621 to 162N and the CLSF 1613.

Thus, by setting the ATM connections between the terminals 1621 to 162Nand the CLSF 1613, and assigning the network ID for the ATM-LAN 1603 tothe CLSF 1613 as well, it becomes possible to realize theconnection-less communication between the ATM-LANs which are logicallyconnected over more than one IWUs.

Next, another embodiment in which the above described ATM communicationsystem according to the present invention is applied for a case ofrealizing a modified network layer topology independent from thetopology of the physical network will be described in detail.

FIG. 67 shows a configuration of the ATM communication system in thisembodiment, which comprises: first, second, and third ATM-LANs 1701,1702, and 1703, where the first and second ATM-LANs 1701 and 1702 areinter-networking through an IWU 1711 while the second and third ATM-LANs1702 and 1703 are inter-networking through an IWU 1712. Here, the firstand second ATM-LANs 1701 and 1702 have CLSFs 1721 and 1722,respectively, whereas the third ATM-LAN 1703 has no CLSF.

The third ATM-LAN 1703 is logically connected with the CLSF 1722 in thesecond ATM-LAN 1702 as indicated by a logical connection 1731 in thesame manner as in the above. Moreover, the third ATM-LAN 1703 is alsologically connected with the CLSF 1721 in the first ATM-LAN 1701 whichis separated by more than one hops in terms of the number of the IWUs asindicated by a logical connection 1732 tunnelling through the IWUs 1711and 1712, as in the case of FIG. 65 described above. Thus, the thirdATM-LAN 1703 logically has two CLSFs.

In this state, a case of executing the RIP described above between theCLSFs 1721 and 1722 will now be described.

The CLSF 1721 has the ATM-LANs 1701 and 1703 as the networks to which itcan make the direct delivery, so that it notifies the CLSF 1722 that"ATM-LANs 1701 and 1703 can be reached by one hop". The CLSF 1722 hasthe ATM-LANs 1702 and 1703 as the networks to which it can make thedirect delivery, so that it notifies the CLSF 1721 that "ATM-LANs 1702and 1703 can be reached by one hop".

By this data exchange, the routing table 1751 is constructed at the CLSF1721 and the routing table 1752 is constructed at the CLSF 1722. Inaddition, the ATM-LAN 1703 eventually receives the routing dataindicating that "ATM-LAN 1701 can be reached by one hop" and "ATM-LAN1702 can be reached by two hops" from the CLSF 1721, as well as therouting data indicating that "ATM-LAN 1701 can be reached by two hops"and "ATM-LAN 1702 can be reached by one hop" from the CLSF 1722. Then,the device 1741 in the ATM-LAN 1703 which is making the passiveoperation can construct the routing table 1753 indicating that "thedatagram destined to the ATM-LAN 1701 should be transmitted to the CLSF1721" and "the datagram destined to the ATM-LAN 1702 should betransmitted to the CLSF 1722" according to these routing data receivedby the ATM-LAN 1703.

As for the other ATM-LANs 1701 and 1702, each of them has only one CLSFconnected, so that there is no choice of the CLSFs available. However,each of them can be modified to provide the choice of the CLSFssimilarly to the ATM-LAN 1703 by providing the logical connectionbetween the ATM-LAN 1702 and the CLSF 1721 or the logical connectionbetween the ATM-LAN 1701 and the CLSF 1722, if desired.

Now, a scheme for setting up the ATM connection between the IWU and theCLSF in this embodiment will be described with reference to FIG. 68. Inthis case, the ATM-LANs 1801 and 1802 are inter-networking through theIWU 1811, and the ATM-LAN 1801 contains the CLSF 1821, while the ATM-LAN1802 is logically connected with the CLSF 1821 in the ATM-LAN 1801 as inthe above.

Between the CLSF 1821 and the IWU 1811, as many ATM connections (whichdefine VCs) 1831 to 183N as a number N of terminals belonging to theATM-LAN 1802 are set up. By bundling these VCs 1831 to 183N together asone VP 1841, it becomes possible for the ATM-LAN 1801 to realize thecell transmission between the CLSF 1821 and the IWU 1811 by only lookingup the 8 bits VPI indicating the VP 1841, without looking up the 16 bitsVCI.

In addition, by directly connecting each connection of the VCs 1831 to183N in the VP 1841 with the already established ATM connection (notshown) between the IWU 1811 and the terminal in the ATM-LAN 1802 at theATM layer by the IWU 1811, the cell transmission from the terminal inthe ATM-LAN 1802 to the CLSF 1821 can be realized as the IWU 1811carries out the relaying of the cell from the ATM connection 1851 to theATM connection given by one of the VCs 1831 to 183N in the VP 1841. Inaddition, for the cells transmitted from the CLSF 1821 to the terminalin the ATM-LAN 1802, the IWU 1811 also carries out the relaying of thecell for the ATM connection between the IWU 1811 and the terminal in theATM-LAN 1802 by looking up the VCI field, to realize the celltransmission between the CLSF 1821 and the terminal in the ATM-LAN 1802in a case of using the bundling of the VCs 1831 to 183N into the VP 841.

Next, a scheme for setting up the ATM connection between the IWU and theCLSF in this embodiment in a case of involving the connection betweenthe ATM-LANs which are separated by more than one hops in terms of thenumber of IWUs will be described with reference to FIG. 69. In thiscase, the ATM-LANs 1901, 1902, and 1903 are provided, where the ATM-LANs1901 and 1902 are inter-networking through the IWU 1911 while theATM-LANs 1902 and 1903 are inter-networking through the IWU 1912, andthe ATM-LAN 1901 contains the CLSF 1921, while the ATM-LANs 1902 and1903 are logically connected with the CLSF 1921 in the ATM-LAN 1901 asin the above.

Between the CLSF 1921 and the IWU 1911, as many ATM connections whichdefine VCs 1931 to 193M as a number M of terminals belonging to theATM-LAN 1902 are set up, along with as many ATM connections which defineVCs 1941 to 194N as a number N of terminals belonging to the ATM-LAN1903.

By bundling these VCs 1931 to 193M and 1941 to 194N together as one VP1941, it becomes possible for the ATM-LAN 1901 to exchange the datagramsdestined to the ATM-LANs 1902 and 1903 which are outputted from the CLSF1921 to the IWU 1911 and the datagrams from the terminals in theATM-LANs 1902 and 1903 by the 8 bits VPI indicating the VP 1951.

In addition, the terminals in the ATM-LAN1902 and the VCs 1931 to 193Mbundled into the VP 1951 set up between the the IWU 1911 and the CLSF1921, as well as the VCs 1961 and 196N set up between the IWUs 1911 and1912 and the VCs 1941 to 194N bundled into the VP 1951 are directlyconnected at the ATM layer by the IWU 1911. For this reason, for the VP1951, the IWU 1911 sets whether it is the tunnelling to the IWU 1912 orthe delivery to the ATM-LAN 1902, according to the VCI values of thosewhich have arrived from the CLSF 1921. Also, for those which havearrived from the terminals in the ATM-LAN 1902, the IWU 1911 is set tocarry out the relaying of the cells to the connection (not shown)between the terminals in the ATM-LAN 1902 and the IWU 1911 and the ATMconnections indicated by the VP 1951 and the VCs 1931 to 193M, and forthose which have arrived from the IWU 1912, the IWU 1911 is set to carryout the relaying of the cells to the VP 1951 and the VCs 1941 to 194N.

By these settings, in a case of involving the connection of the ATM-LANswhich are separated by more than one hops in terms of the number ofIWUs, the ATM connections between the CLSF 1921 and the IWU 1911 can bebundled together.

Similarly, in the ATM-LAN 1902, for those which are to be tunnelled tothe IWU 1912 among the above described VCs, by bundling the VCs 1961 to196N in the ATM-LAN 1902 into one VP 1952, they can be exchanged withthe IWU 1912 by only looking up the 8 bits VPI indicating the VP 1952.

Next, a method for judging whether it is the tunnelling to the IWU 1912or the delivery to the terminals in the ATM-LAN 1902 will be described.

Namely, in an exemplary case of the cell header 1971 shown in FIG. 70,the VPI indicates the route IWU 1911 → CLSF 1921 or the route CLSF 1921→ IWU 1911. Here, these routes may be indicates by the same VPI value orby the different VPI values.

The upper P bits 1972 of the VCI indicating the VCs 1931 to 193M and theVCs 1941 to 194N between the IWU 1911 and the CLSF 1921 are used as thenetwork identification, and set up such that it is possible to judgewhether it is the delivery to the terminals in the ATM-LAN 1902 or thetunnelling to the IWU 1912 according to the value of these VCI upper Pbits 1972 at the IWU 1911. In a case of the delivery to the ATM-LAN1902, they are converted into the VCI/VPI between the IWU 1911 and theterminals in the ATM-LAN 1902 by the VCI lower (16-P) bits 1973 and thecell is outputted to the terminals. Namely, the transmission targetterminal is identified according to the VCI lower (16-P) bits 1973. In acase of the tunnelling to the IWU 1912, the cell is outputted to the VPfor the tunnelling to the IWU 1912. Here, for the VCI value, any of (1)changing only the upper P bits of the VCI, (2) changing all of the VCI,and (3) not changing the VCI, can be selected appropriately.

For the cell arriving from the terminals in the ATM-LAN 1902, therelaying of the cells to the ATM connections indicated by the VCs 1931to 193M in the VP 1951 is carried out at the IWU 1911. In this case, theATM connections set up between the IWU 1911 and the terminals in theATM-LAN 1902 and the VCs 1931 to 193M are directly connected in the IWU1911. At this point, the header value to be assigned to the VCs 1931 to193N may be the same value or the different value as the value of theATM connection from the CLSF 1921 to the IWU 1911.

As for the cell transmitted from the IWU 1912 to the IWU 1911, therelaying of the cells to the ATM connections indicated by the VCs 1941to 194N in the VP 1951 is carried out at the IWU 1911. At this point,the value of the VCs 1941 to 194N may be the same value or the differentvalue as the value of the ATM connection from the CLSF 1921 to the IWU1911.

As for the cells which are tunnelled, by using the format of the cellheader 1971 in the ATM-LAN 1902, and carrying out the same processing asthe IWU 1911 at the IWU 1912, the IWU can pass the cell by the sameprocessing for the networks which are separated by more than two hops interms of the number of IWUs.

Here, the VCI upper P bits 1972 can have an optional number of bits P,and the number of ATM-LANs that can be contained and the number ofterminals in the ATM-LAN without the CLSF are varied according to thevalue of P. In a case the connections between the IWU and the CLSF arebundled into one VP, the number of terminals to which the CLSF can makethe direct delivery is going to be 2¹⁶ regardless of the value of P.

Next, a method of carrying out the broadcast between the CLSF and theATM-LANs which are logically connected with CLSF as in the above will bedescribed with reference to FIG. 71.

In FIG. 71, the ATM-LANs 1001 and 1002 are inter-networking through theIWU 1011, and the ATM-LAN 1001 contains the CLSF 1021, while the ATM-LAN1002 is logically connected with the CLSF 1021 as in the above, and asmany ATM connections 1031 as the number of terminals belonging to theATM-LAN 1002 are provided as the ATM connections between the CLSF 1021and the IWU 1011. In addition, the ATM connection 1041 for broadcast isalso provided, and this ATM connection 1041 is directly connected at theATM layer to the broadcast channel at the ATM-LAN 1002.

In a case the CLSF 1021 makes the broadcast with respect to the ATM-LAN1002, first the CLSF 1021 transmits the broadcast cell to the ATMconnection 1041 for broadcast. This ATM connection 1041 is not for thebroadcast in the ATM-LAN 1001, so that the broadcast is not carried outin the ATM-LAN 1001. At the IWU 1011, the cells transmitted from the ATMconnection 1041 for broadcast are outputted to the broadcast channel ofthe ATM-LAN 1002. By this, the the cells in the ATM-LAN 1002 isbroadcasted through the broadcast channel in the ATM-LAN 1002.

On the other hand, as for the broadcast cells generated from theterminals in the ATM-LAN 1002, the broadcast cells may be made to arriveat the CLSF 1021 as well by outputting them to the ATM connection 1041for broadcast at the IWU 1011.

By this method, the transmission and reception of the broadcast cellsbetween the CLSF 1021 and the ATM-LAN 1002 can be realized withoutloading the ATM-LAN 1001 which contains the CLSF 1021.

Next, a method of carrying out the broadcast between the CLSF and theATM-LANs which are logically connected with CLSF and separated by morethan one hops in terms of the number of IWUs will be described withreference to FIG. 72.

In FIG. 72, the ATM-LANs 2101 and 2102 are inter-networking through theIWU 2111, and the ATM-LANs 2102 and 2103 are inter-networking though theIWU 2112. The ATM-LAN 2101 contains the CLSF 2121, while the ATM-LANs2102 and 2103 are logically connected with the CLSF 2121 as in theabove.

Between the CLSF 2121 and the IWU 2111, as many ATM connections (notshown) as the number of terminals belonging to the ATM-LAN 2102 are setup, and between the CLSF 2121 and the IWU 2111 and between the IWU 2111and the IWU 2112, as many ATM connections 2131 and 2141, respectively,as the number of the terminals belonging to the ATM-LAN 2103 are set up,where the ATM connections 2131 and 2141 are directly connected at theATM layer by the IWU 2111.

In addition, the ATM connection 2161 for broadcast is provided betweenthe IWUs 2111 and 2112, and the ATM connection 2151 for broadcast isprovided between the IWU 2111 and the CLSF 2121, where these ATMconnections 2161 and 2151 are directly connected at the ATM layer by theIWU 2111. Also, the ATM connection 2161 for the broadcast in the ATM-LAN2103 is directly connected with the broadcast channel in the ATM-LAN2103 at the ATM layer by the IWU 2112.

In a case the CLSF 2121 makes the broadcast with respect to the ATM-LAN2103, the CLSF 2121 transmits the broadcast cell to the IWU 2111 throughthe ATM connection 2151 for broadcast in the ATM-LAN 2103. Here, thebroadcast is not made in the ATM-LAN 2101 as there is no broadcastchannel in the ATM-LAN 2101.

At the IWU 2111, the relaying of the cells to the IWU 2112 is carriedout by using the ATM connection 2161 for broadcast in the ATM-LAN 2103.At the IWU 2112, the cells transmitted from the ATM connection 2161 forbroadcast are outputted to the broadcast channel of the ATM-LAN 2103, soas to realize the broadcast through the broadcast channel in the ATM-LAN2103.

On the other hand, as for the cells broadcasted at the terminals in theATM-LAN 2103, the cells are transmitted to the IWU 2111 by using the ATMconnection 2161 for broadcast which is set up between the IWUs 2112 and2111 by the IWU 2112. In addition, the cells may be transmitted to theCLSF 2121 by using the ATM connection 2151 for broadcast which i set upbetween the IWU 2111 and the CLSF 2121 by the IWU 2111.

By this method, the transmission and reception of the broadcast cellscan be realized without loading the CLSF 2121, the ATM-LAN 2101 in whichthe CLSF 2121 is contained, and the ATM-LAN 2102 to be tunnelled.

It is noted that, in the above description, the state of being directlyconnected at the ATM layer can be equivalently expressed as a state ofbeing able to transmit the cell without carrying out the AAL (ATMAdaptation Layer) processing. In addition, in the above description, theaddress data at the network layer can be replaced by the address data atthe CL (Connection-Less) layer, i.e., the upper layer of the AAL ifdesired.

It is to be noted here that the embodiments for realizing the modifiednetwork layer topology described above can be combined with theembodiments for the datagram delivery in the hierarchical or flattopology described earlier, by using the logically connected CLSF as theCLSF associated with the terminals belonging to the network in which noCLSF is provided physically, in place of the CLSF provided in eachnetwork in the datagram delivery schemes described above.

It is also to be noted here that, besides those already mentioned above,many modifications and variations of the above embodiments may be madewithout departing from the novel and advantageous features of thepresent invention. Accordingly, all such modifications and variationsare intended to be included within the scope of the appended claims.

What is claimed is:
 1. A network system comprising:a first networkhaving a first connection-less service function unit; a second networkhaving a second connection-less service function unit; a data transferunit for specifying a first connection for a transfer at a layer lowerthan a network layer from the data transfer unit through the first andsecond networks to the second connection-less service function unit andfor transferring a datagram onto the first connection when the datagramis to be transferred from a source terminal located on or beyond thefirst network from a viewpoint of the second network, through the firstnetwork to a destination terminal located on or beyond the secondnetwork from a viewpoint of the first network.
 2. The system of claim 1,wherein the data transfer unit transfers a datagram onto a secondconnection connected to the first connection-less service function unitwhen the datagram is to be transferred to the first network.
 3. Thesystem of claim 1, wherein the data transfer unit specifies the firstconnection to be used for transferring the datagram according to atleast a destination of the datagram.
 4. The system of claim 1, furthercomprising:a third network having a third connection-less servicefunction unit, wherein the data transfer unit specifies a secondconnection for a transfer at said layer lower than the network layerfrom the data transfer unit through the first and third networks to thethird connection-less service function unit and transfers a datagramonto said second connection when the datagram is to be transferredthrough the first network to or beyond the third network from aviewpoint of the first network.
 5. The system of claim 1, furthercomprising an inter-networking unit located between the first and secondnetworks,wherein the data transfer unit specifies the first connectionwhich is set up, through the inter-networking unit, from a thirdconnection-less service function unit located on a source side of thedatagram to the second connection-less service function unit.
 6. Thesystem of claim 5, wherein the inter-networking unit is integrallyprovided with the first connection-less service function unit.
 7. Thesystem of claim 5, wherein the inter-networking unit switchesconnections through the first and second networks on said layer lowerthan the network layer.
 8. The system of claim 1, wherein each of thefirst and second connection-less service function units determines acorresponding connection onto which a datagram is to be transferred byanalyzing the datagram.
 9. The system of claim 1, wherein the datatransfer unit specifies the first connection according to acorrespondence between a destination and a connection identifierprovided in advance.
 10. The system of claim 1, further comprising aninter-networking unit located between the first and secondnetworks,wherein the data transfer unit specifies the first connectionwhich is set up, through the inter-networking unit, from the datatransfer unit to the second connection-less service function unit.
 11. Amethod of data transfer in a network system including a first networkhaving a first connection-less service function unit and a secondnetwork having a second connection-less service function unit, themethod comprising the steps of:specifying a first connection for atransfer at a layer lower than a network layer from the data transferunit through the first and second networks to the second connection-lessservice function unit when a datagram is to be transferred from a sourceterminal located on or beyond the first network from a viewpoint of thesecond network, through the first network to a destination terminallocated on or beyond the second network from a viewpoint of the firstnetwork; and transferring the datagram onto the first connectionspecified in the specifying step.
 12. The method of claim 11, furthercomprising the step of:transferring a datagram onto a second connectionconnected to the first connection-less service function unit when thedatagram is to be transferred to the first network.
 13. The method ofclaim 11, wherein the specifying step specifies the first connection tobe used for transferring the datagram according to at least adestination of the datagram.
 14. The method of claim 11, wherein thenetwork system further includes a third network having a thirdconnection-less service function unit, andthe specifying step specifiesa second connection for a transfer at said layer lower than the networklayer from the data transfer unit through the first and third networksto the third connection-less service function unit and the transferringstep transfers a datagram onto said second connection, when the datagramis to be transferred through the first network to or beyond the thirdnetwork from a viewpoint of the first network.
 15. The method of claim11, wherein the network system further includes an inter-networking unitlocated between the first and second networks, andthe specifying stepspecifies the first connection which is set up, through theinter-networking unit, from a third connection-less service functionunit located on a source side of the datagram to the secondconnection-less service function unit.
 16. The method of claim 11,further comprising the step of:determining a corresponding connectiononto which a datagram is to be transferred by analyzing the datagram ateach of the first and second connection-less service function units. 17.The method of claim 11, wherein the specifying step specifies the firstconnection according to a correspondence between a destination and aconnection identifier provided in advance.
 18. The method of claim 11,wherein the network system further includes an inter-networking unitlocated between the first and second networks, andthe specifying stepspecifies the first connection which is set up, through theinter-networking unit, from a data transfer apparatus that carries outthe specifying step and the transferring step to the secondconnection-less service function unit.
 19. A data transfer apparatus ina network system including a first network having a firstconnection-less service function unit and a second network having asecond connection-less service function unit, the apparatus beinglocated on or beyond the first network from a viewpoint of the secondnetwork and comprising:a connection specifying unit for specifying afirst connection for a transfer at a layer lower than the network layerfrom the data transfer apparatus through the first and second networksto the second connection-less service function unit when a datagram isto be transferred from a source terminal located on or beyond the firstnetwork from a viewpoint of the second network, through the firstnetwork to a destination terminal located on or beyond the secondnetwork from a viewpoint of the first network; and a datagramtransferring unit for transferring the datagram onto the firstconnection specified by the connection specifying unit.
 20. Theapparatus of claim 19, wherein the datagram transferring unit transfersa datagram which is to be transferred to the first network onto a secondconnection connected to the first connection-less service function unit.21. The apparatus of claim 19, wherein the connection specifying unitspecifies the first connection to be used by the datagram transferringunit according to at least a destination of the datagram.
 22. Theapparatus of claim 19, wherein the network system further includes athird network having a third connection-less service function unit,andthe connection specifying unit also specifies a second connection fora transfer at said layer lower than the network layer from the datatransfer apparatus through the first and third networks to the thirdconnection-less service function unit so that the datagram transferringunit transfers a datagram onto said second connection when the datagramis to be transferred through the first network to or beyond the thirdnetwork from a viewpoint of the first network.
 23. The apparatus ofclaim 19, wherein the network system further includes aninter-networking unit located between the first and second networks,andthe connection specifying unit specifies the first connection whichis set up, through the inter-networking unit, from a thirdconnection-less service function unit located on a source side of thedatagram to the second connection-less service function unit.
 24. Theapparatus of claim 19, wherein the connection specifying unit specifiesthe first connection according to a correspondence between a destinationand a connection identifier provided in advance.
 25. The apparatus ofclaim 19, wherein the network system further includes aninter-networking unit located between the first and second networks,andthe connection specifying unit specifies the first connection whichis set up, through the inter-networking unit, from the data transferapparatus to the second connection-less service function unit.